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

ci/build-tarballs.sh

@@ -48,7 +48,7 @@ mkdir tmp/$api_pkgname/lib
 mkdir tmp/$api_pkgname/include
 cp LICENSE README.md tmp/$api_pkgname
 mv bins-$src/* tmp/$api_pkgname/lib
-cp crates/api/c-examples/wasm-c-api/include/wasm.h tmp/$api_pkgname/include
+cp crates/c-api/examples/wasm-c-api/include/wasm.h tmp/$api_pkgname/include
 mktarball $api_pkgname
 
 # Move wheels to dist folder