combine fiber abstractions from async_.rs and concurrent.rs

A long time ago, when I first started implementing CM async, I cut-and-pasted
`async_.rs`'s `FiberFuture` and heavily modified it.  Both have evolved
separately since then, and now that the CM async work has generally stabilized
it's time to unify them.

This creates a new `runtime::fiber` submodule which `async_.rs` and
`concurrent.rs` share.  The central type is `StoreFiber`, which is a more
versatile version of `FiberFuture`, supporting fibers which can either retain
exclusive access to the store across suspend points or release it, allowing the
store to be used by the root event loop and/or other fibers to run before the
original fiber resumes.

Signed-off-by: Joel Dice <joel.dice@fermyon.com>

Author: dicej

.gitignore

@@ -28,6 +28,7 @@ cranelift/isle/veri/veri_engine/test_output
 crates/explorer/node_modules
 tests/all/pulley_provenance_test.cwasm
 tests/all/pulley_provenance_test_component.cwasm
+tests/all/pulley_provenance_test_async_component.cwasm
 /artifacts
 testcase*.wat
 testcase*.wasm

crates/wasmtime/src/runtime.rs

@@ -34,6 +34,8 @@ pub(crate) mod code_memory;
 #[cfg(feature = "debug-builtins")]
 pub(crate) mod debug;
 pub(crate) mod externals;
+#[cfg(feature = "async")]
+pub(crate) mod fiber;
 pub(crate) mod gc;
 pub(crate) mod instance;
 pub(crate) mod instantiate;