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

crates/api/src/runtime.rs

@@ -398,7 +398,13 @@ impl Store {
             .cloned()
     }
 
-    pub(crate) fn ptr_eq(a: &Store, b: &Store) -> bool {
+    /// Returns whether the stores `a` and `b` refer to the same underlying
+    /// `Store`.
+    ///
+    /// Because the `Store` type is reference counted multiple clones may point
+    /// to the same underlying storage, and this method can be used to determine
+    /// whether two stores are indeed the same.
+    pub fn same(a: &Store, b: &Store) -> bool {
         Rc::ptr_eq(&a.inner, &b.inner)
     }
 }