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/debug/src/transform/expression.rs

@@ -1,6 +1,4 @@
 use super::address_transform::AddressTransform;
-use crate::{HashMap, HashSet};
-use alloc::vec::Vec;
 use anyhow::Error;
 use cranelift_codegen::ir::{StackSlots, ValueLabel, ValueLoc};
 use cranelift_codegen::isa::RegUnit;
@@ -9,6 +7,7 @@ use cranelift_entity::EntityRef;
 use cranelift_wasm::{get_vmctx_value_label, DefinedFuncIndex};
 use gimli::{self, write, Expression, Operation, Reader, ReaderOffset, Register, X86_64};
 use more_asserts::{assert_le, assert_lt};
+use std::collections::{HashMap, HashSet};
 
 #[derive(Debug)]
 pub struct FunctionFrameInfo<'a> {
@@ -201,7 +200,7 @@ impl CompiledExpression {
         addr_tr: &AddressTransform,
         frame_info: Option<&FunctionFrameInfo>,
         endian: gimli::RunTimeEndian,
-    ) -> alloc::vec::Vec<(write::Address, u64, write::Expression)> {
+    ) -> Vec<(write::Address, u64, write::Expression)> {
         if scope.is_empty() {
             return vec![];
         }