This is a mix of exposing new things (e.g. a `Table::fill` method) and extending existing support to `externref`s (e.g. `Table::new`). Part of #929
This is the wasmtime-runtime crate, which contains wasm runtime library
support, supporting the wasm ABI used by wasmtime-environ,
wasmtime-jit, and wasmtime-obj.
This crate does not make a host vs. target distinction; it is meant to be compiled for the target.
Most users will want to use the main wasmtime crate instead of using this
crate directly.