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.
This moves the Wasmtime for .NET implementation to the Wasmtime repo. Wasmtime for .NET is a binding of the Wasmtime API for use in .NET.