58bb5dd953
`funcref`s are implemented as `NonNull<VMCallerCheckedAnyfunc>`. This should be more efficient than using a `VMExternRef` that points at a `VMCallerCheckedAnyfunc` because it gets rid of an indirection, dynamic allocation, and some reference counting. Note that the null function reference is *NOT* a null pointer; it is a `VMCallerCheckedAnyfunc` that has a null `func_ptr` member. Part of #929
This is the wasmtime-jit crate, which contains JIT-based execution
for wasm, using the wasm ABI defined by wasmtime-environ and the
runtime support provided by wasmtime-runtime.
Most users will want to use the main wasmtime crate instead of using this
crate directly.