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/environ/src/cache/tests.rs

@@ -3,15 +3,13 @@ use super::*;
 use crate::address_map::{FunctionAddressMap, InstructionAddressMap};
 use crate::compilation::{CompiledFunction, Relocation, RelocationTarget, TrapInformation};
 use crate::module::{MemoryPlan, MemoryStyle, Module};
-use alloc::boxed::Box;
-use alloc::vec::Vec;
-use core::cmp::min;
 use cranelift_codegen::{binemit, ir, isa, settings, ValueLocRange};
 use cranelift_entity::EntityRef;
 use cranelift_entity::{PrimaryMap, SecondaryMap};
 use cranelift_wasm::{DefinedFuncIndex, FuncIndex, Global, GlobalInit, Memory, SignatureIndex};
 use rand::rngs::SmallRng;
 use rand::{Rng, SeedableRng};
+use std::cmp::min;
 use std::fs;
 use std::str::FromStr;
 use target_lexicon::triple;