Migrate back to std:: stylistically (#554)

* Migrate back to `std::` stylistically

This commit moves away from idioms such as `alloc::` and `core::` as
imports of standard data structures and types. Instead it migrates all
crates to uniformly use `std::` for importing standard data structures
and types. This also removes the `std` and `core` features from all
crates to and removes any conditional checking for `feature = "std"`

All of this support was previously added in #407 in an effort to make
wasmtime/cranelift "`no_std` compatible". Unfortunately though this
change comes at a cost:

* The usage of `alloc` and `core` isn't idiomatic. Especially trying to
  dual between types like `HashMap` from `std` as well as from
  `hashbrown` causes imports to be surprising in some cases.
* Unfortunately there was no CI check that crates were `no_std`, so none
  of them actually were. Many crates still imported from `std` or
  depended on crates that used `std`.

It's important to note, however, that **this does not mean that wasmtime
will not run in embedded environments**. The style of the code today and
idioms aren't ready in Rust to support this degree of multiplexing and
makes it somewhat difficult to keep up with the style of `wasmtime`.
Instead it's intended that embedded runtime support will be added as
necessary. Currently only `std` is necessary to build `wasmtime`, and
platforms that natively need to execute `wasmtime` will need to use a
Rust target that supports `std`. Note though that not all of `std` needs
to be supported, but instead much of it could be configured off to
return errors, and `wasmtime` would be configured to gracefully handle
errors.

The goal of this PR is to move `wasmtime` back to idiomatic usage of
features/`std`/imports/etc and help development in the short-term.
Long-term when platform concerns arise (if any) they can be addressed by
moving back to `no_std` crates (but fixing the issues mentioned above)
or ensuring that the target in Rust has `std` available.

* Start filling out platform support doc

crates/runtime/src/vmcontext.rs

@@ -2,8 +2,8 @@
 //! fields that compiled wasm code accesses directly.
 
 use crate::instance::Instance;
-use core::any::Any;
-use core::{ptr, u32};
+use std::any::Any;
+use std::{ptr, u32};
 use wasmtime_environ::BuiltinFunctionIndex;
 
 /// An imported function.
@@ -20,8 +20,8 @@ pub struct VMFunctionImport {
 #[cfg(test)]
 mod test_vmfunction_import {
     use super::VMFunctionImport;
-    use core::mem::size_of;
     use memoffset::offset_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -53,7 +53,7 @@ pub struct VMFunctionBody(u8);
 #[cfg(test)]
 mod test_vmfunction_body {
     use super::VMFunctionBody;
-    use core::mem::size_of;
+    use std::mem::size_of;
 
     #[test]
     fn check_vmfunction_body_offsets() {
@@ -76,8 +76,8 @@ pub struct VMTableImport {
 #[cfg(test)]
 mod test_vmtable_import {
     use super::VMTableImport;
-    use core::mem::size_of;
     use memoffset::offset_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -114,8 +114,8 @@ pub struct VMMemoryImport {
 #[cfg(test)]
 mod test_vmmemory_import {
     use super::VMMemoryImport;
-    use core::mem::size_of;
     use memoffset::offset_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -149,8 +149,8 @@ pub struct VMGlobalImport {
 #[cfg(test)]
 mod test_vmglobal_import {
     use super::VMGlobalImport;
-    use core::mem::size_of;
     use memoffset::offset_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -184,8 +184,8 @@ pub struct VMMemoryDefinition {
 #[cfg(test)]
 mod test_vmmemory_definition {
     use super::VMMemoryDefinition;
-    use core::mem::size_of;
     use memoffset::offset_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -228,8 +228,8 @@ pub struct VMTableDefinition {
 #[cfg(test)]
 mod test_vmtable_definition {
     use super::VMTableDefinition;
-    use core::mem::size_of;
     use memoffset::offset_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -265,8 +265,8 @@ pub struct VMGlobalDefinition {
 #[cfg(test)]
 mod test_vmglobal_definition {
     use super::VMGlobalDefinition;
-    use core::mem::{align_of, size_of};
     use more_asserts::assert_ge;
+    use std::mem::{align_of, size_of};
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -432,7 +432,7 @@ pub struct VMSharedSignatureIndex(u32);
 #[cfg(test)]
 mod test_vmshared_signature_index {
     use super::VMSharedSignatureIndex;
-    use core::mem::size_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, TargetSharedSignatureIndex, VMOffsets};
 
     #[test]
@@ -485,8 +485,8 @@ pub struct VMCallerCheckedAnyfunc {
 #[cfg(test)]
 mod test_vmcaller_checked_anyfunc {
     use super::VMCallerCheckedAnyfunc;
-    use core::mem::size_of;
     use memoffset::offset_of;
+    use std::mem::size_of;
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]
@@ -559,7 +559,7 @@ pub struct VMInvokeArgument([u8; 16]);
 #[cfg(test)]
 mod test_vm_invoke_argument {
     use super::VMInvokeArgument;
-    use core::mem::{align_of, size_of};
+    use std::mem::{align_of, size_of};
     use wasmtime_environ::{Module, VMOffsets};
 
     #[test]