Move the C API to a separate crate (#818)

* Move the C API to a separate crate

This commit moves the C API from `crates/api/src/wasm.rs` to
`crates/capi/src/lib.rs` to be located in a separate crate. There's a
number of reasons for this:

* When a Rust program depends on the `wasmtime` crate, there's no need
  to compile in the C API.
* This should improve compile times of the `wasmtime` crate since it's
  not producing artifacts which aren't always used.
* The development of the C API can be guaranteed to only use the public
  API of the `wasmtime` crate itself.

Some CI pieces are tweaked and this overall shouldn't have much impact
on users, it's intended that it's a cleanup/speedup for developers!

* Disable rustdoc/tests for capi

* Review feedback

* Add back in accidentally deleted comment

* More renamings

* Try to fix dotnet build

.github/workflows/main.yml

@@ -302,7 +302,7 @@ jobs:
     - run: $CENTOS cargo build --release --bin wasmtime --bin wasm2obj
       shell: bash
     # Build `libwasmtime.so`
-    - run: $CENTOS cargo build --release --manifest-path crates/api/Cargo.toml
+    - run: $CENTOS cargo build --release --manifest-path crates/c-api/Cargo.toml
       shell: bash
     # Test what we just built
     - run: $CENTOS cargo test --features test_programs --release --all --exclude lightbeam --exclude wasmtime --exclude wasmtime-fuzzing