This commit makes `WasiCtxBuilder` take `&mut Self` and return `&mut
Self` for its methods. This is needed to allow for the same
(unmoved) `WasiCtxBuilder` to be used when building a WASI context.
Also fixes up the C API to remove the unnecessary `Box::from_raw` and
`forget` calls which were previously needed for the moving version of
`WasiCtxBuilder`.
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.
