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

docs/stability-platform-support.md

@@ -1,3 +1,43 @@
 # Platform Support
 
-... more coming soon
+The `wasmtime` project is a configurable and lightweight runtime for WebAssembly
+which has a number of ways it can be configured. Not all features are supported
+on all platforms, but it is intended that `wasmtime` can run in some capacity on
+almost all platforms! The matrix of what's being tested, what works, and what's
+supported where is evolving over time, and this document hopes to capture a
+snapshot of what the current state of the world looks like.
+
+All features of `wasmtime` should work on the following platforms:
+
+* Linux x86\_64
+* macOS x86\_64
+* Windows x86\_64
+
+For more detailed information about supported platforms, please check out the
+sections below!
+
+## JIT compiler support
+
+The JIT compiler, backed by either `lightbeam` or `cranelift` supports only the
+x86\_64 architecture at this time. Support for at least ARM, AArch64, and x86 is
+planned at this time.
+
+Usage of the JIT compiler will require a host operating system which supports
+creating executable memory pages on-the-fly. In Rust terms this generally means
+that `std` needs to be supported on this platform.
+
+## Interpreter support
+
+At this time `wasmtime` does not have a mode in which it simply interprets
+WebAssembly code. It is planned to add support for an interpreter, however, and
+this will have minimal system dependencies. It is planned that the system will
+need to support some form of dynamic memory allocation, but other than that not
+much else will be needed.
+
+## What about `#[no_std]`?
+
+The `wasmtime` project does not currently use `#[no_std]` for its crates, but
+this is not because it won't support it! At this time we're still gathering use
+cases for for what `#[no_std]` might entail, so if you're interested in this
+we'd love to hear about your use case! Feel free to open an issue on the
+`wasmtime` repository to discuss this.