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/jit/src/compiler.rs

@@ -1,13 +1,8 @@
 //! JIT compilation.
 
-use super::HashMap;
 use crate::code_memory::CodeMemory;
 use crate::instantiate::SetupError;
 use crate::target_tunables::target_tunables;
-use alloc::boxed::Box;
-use alloc::string::String;
-use alloc::vec::Vec;
-use core::convert::TryFrom;
 use cranelift_codegen::ir::InstBuilder;
 use cranelift_codegen::isa::{TargetFrontendConfig, TargetIsa};
 use cranelift_codegen::print_errors::pretty_error;
@@ -16,6 +11,8 @@ use cranelift_codegen::{binemit, ir};
 use cranelift_entity::{EntityRef, PrimaryMap};
 use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext};
 use cranelift_wasm::{DefinedFuncIndex, DefinedMemoryIndex, ModuleTranslationState};
+use std::collections::HashMap;
+use std::convert::TryFrom;
 use wasmtime_debug::{emit_debugsections_image, DebugInfoData};
 use wasmtime_environ::{
     Compilation, CompileError, CompiledFunction, Compiler as _C, FunctionBodyData, Module,
@@ -202,7 +199,7 @@ impl Compiler {
         signature: &ir::Signature,
         value_size: usize,
     ) -> Result<*const VMFunctionBody, SetupError> {
-        use super::hash_map::Entry::{Occupied, Vacant};
+        use std::collections::hash_map::Entry::{Occupied, Vacant};
         Ok(match self.trampoline_park.entry(callee_address) {
             Occupied(entry) => *entry.get(),
             Vacant(entry) => {