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-environ crate, which contains the implementations
of the ModuleEnvironment and FuncEnvironment traits from
cranelift-wasm. They effectively
implement an ABI for basic wasm compilation that defines how linear memories
are allocated, how indirect calls work, and other details. They can be used
for JITing, native object files, or other purposes.