This commit implements an initial WASI C API that can be used to instantiate
and configure a WASI instance from C.
This also implements a `WasiBuilder` for the C# API enabling .NET hosts to bind
to Wasmtime's WASI implementation.
It appears there are two trailing null bytes at the end of the string.
This does not seem right. But it might be a good idea generally to remove
any null bytes that get into error messages.
