Move this repository into the upstream Wasmtime repository

[alexcrichton]

I'd like to float the idea of moving these bindings (aka this header file) from this repository into upstream bytecodealliance/wasmtime. That would enable I think more easily testing the C API in Wasmtime itself without placing too much burden on authors of the C API. Beyond that I don't have motivation, though, so I'm curious what folks think about this.

[alexcrichton]

I've added this idea to the Wasmtime meeting agenda for next week.