Start migrating some Wasmtime crates to no_std

[alexcrichton]

This commit is the first in what will be multiple PRs to migrate Wasmtime to being compatible with #![no_std]. This work is outlined in #8341 and the rough plan I have in mind is to go on a crate-by-crate basis and use CI as a "ratchet" to ensure that no_std compat is preserved. In that sense this PR is a bit of a template for future PRs.

This PR migrates a few small crates to no_std, basically those that need no changes beyond simply adding the attribute. The nontrivial parts introduced in this PR are:

• CI is introduced to verify that a subset of crates can indeed be built on a no_std target. The target selected is x86_64-unknown-none which is known to not have std and will result in a build error if it's attempted to be used.

• The anyhow crate, which wasmtime-jit-icache-coherence now depends on, has its std feature disabled by default in Wasmtime's workspace. This means that some crates which require std now need to explicitly enable the feature, but it means that by-default its usage is appropriate for no_std.

The first point should provide CI checks that compatibility with no_std indeed works, at least from an "it compiles" perspective. Note that it's not sufficient to test with a target like x86_64-unknown-linux-gnu because extern crate std will work on that target, even when #![no_std] is active.

The second point however is likely to increase maintenance burden in Wasmtime unfortunately. Namely we'll inevitably, either here or in the future, forget to turn on some feature for some crate that's not covered in CI checks. While I've tried to do my best here in covering it there's no guarantee that everything will work and the combinatorial explosion of what could be checked in CI can't all be added to CI. Instead we'll have to rely on bug fixes, users, and perhaps point releases to add more use cases to CI over time as we see fit.

[alexcrichton]

I'll also note that, procedurally, my plan is to discuss #8341 in the Wasmtime meeting tomorrow and assuming no objections start landing PRs, but the goal here isn't to jump the gun by any means but rather just be prepared.