Add a cranelift compile-time feature to wasmtime (#3206)

* Remove unnecessary into_iter/map

Forgotten from a previous refactoring, this variable was already of the
right type!

* Move `wasmtime_jit::Compiler` into `wasmtime`

This `Compiler` struct is mostly a historical artifact at this point and
wasn't necessarily pulling much weight any more. This organization also
doesn't lend itself super well to compiling out `cranelift` when the
`Compiler` here is used for both parallel iteration configuration
settings as well as compilation.

The movement into `wasmtime` is relatively small, with
`Module::build_artifacts` being the main function added here which is a
merging of the previous functions removed from the `wasmtime-jit` crate.

* Add a `cranelift` compile-time feature to `wasmtime`

This commit concludes the saga of refactoring Wasmtime and making
Cranelift an optional dependency by adding a new Cargo feature to the
`wasmtime` crate called `cranelift`, which is enabled by default.

This feature is implemented by having a new cfg for `wasmtime` itself,
`cfg(compiler)`, which is used wherever compilation is necessary. This
bubbles up to disable APIs such as `Module::new`, `Func::new`,
`Engine::precompile_module`, and a number of `Config` methods affecting
compiler configuration. Checks are added to CI that when built in this
mode Wasmtime continues to successfully build. It's hoped that although
this is effectively "sprinkle `#[cfg]` until things compile" this won't
be too too bad to maintain over time since it's also an use case we're
interested in supporting.

With `cranelift` disabled the only way to create a `Module` is with the
`Module::deserialize` method, which requires some form of precompiled
artifact.

Two consequences of this change are:

* `Module::serialize` is also disabled in this mode. The reason for this
  is that serialized modules contain ISA/shared flags encoded in them
  which were used to produce the compiled code. There's no storage for
  this if compilation is disabled. This could probably be re-enabled in
  the future if necessary, but it may not end up being all that necessary.

* Deserialized modules are not checked to ensure that their ISA/shared
  flags are compatible with the host CPU. This is actually already the
  case, though, with normal modules. We'll likely want to fix this in
  the future using a shared implementation for both these locations.

Documentation should be updated to indicate that `cranelift` can be
disabled, although it's not really the most prominent documentation
because this is expected to be a somewhat niche use case (albeit
important, just not too common).

* Always enable cranelift for the C API

* Fix doc example builds

* Fix check tests on GitHub Actions

.github/workflows/main.yml

@@ -75,7 +75,7 @@ jobs:
         echo "${{ runner.tool_cache }}/mdbook/bin" >> $GITHUB_PATH
         cargo install --root ${{ runner.tool_cache }}/mdbook --version ${{ env.CARGO_MDBOOK_VERSION }} mdbook
     - run: (cd docs && mdbook build)
-    - run: cargo build -p wasmtime-wasi --features wasmtime/wat
+    - run: cargo build -p wasmtime-wasi --features wasmtime/wat,wasmtime/cranelift
     - run: (cd docs && mdbook test -L ../target/debug/deps)
 
     # Build Rust API documentation
@@ -124,18 +124,20 @@ jobs:
     - uses: ./.github/actions/install-rust
 
     # Check some feature combinations of the `wasmtime` crate
-    - run: cargo check --manifest-path crates/wasmtime/Cargo.toml --no-default-features
-    - run: cargo check --manifest-path crates/wasmtime/Cargo.toml --features wat
-    - run: cargo check --manifest-path crates/wasmtime/Cargo.toml --features lightbeam
-    - run: cargo check --manifest-path crates/wasmtime/Cargo.toml --features jitdump
-    - run: cargo check --manifest-path crates/wasmtime/Cargo.toml --features cache
-    - run: cargo check --manifest-path crates/wasmtime/Cargo.toml --features async
-    - run: cargo check --manifest-path crates/wasmtime/Cargo.toml --features uffd
+    - run: cargo check -p wasmtime --no-default-features
+    - run: cargo check -p wasmtime --no-default-features --features wat
+    - run: cargo check -p wasmtime --no-default-features --features lightbeam
+    - run: cargo check -p wasmtime --no-default-features --features jitdump
+    - run: cargo check -p wasmtime --no-default-features --features cache
+    - run: cargo check -p wasmtime --no-default-features --features async
+    - run: cargo check -p wasmtime --no-default-features --features uffd
+    - run: cargo check -p wasmtime --no-default-features --features cranelift
+    - run: cargo check -p wasmtime --no-default-features --features cranelift,wat,async,cache
 
     # Check some feature combinations of the `wasmtime-c-api` crate
-    - run: cargo check --manifest-path crates/c-api/Cargo.toml --no-default-features
-    - run: cargo check --manifest-path crates/c-api/Cargo.toml --features wat
-    - run: cargo check --manifest-path crates/c-api/Cargo.toml --features wasi
+    - run: cargo check -p wasmtime-c-api --no-default-features
+    - run: cargo check -p wasmtime-c-api --no-default-features --features wat
+    - run: cargo check -p wasmtime-c-api --no-default-features --features wasi
 
     # Check a few builds of the cranelift backend
     # - only x86 backend support,