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.
The Validate function already checks that the field type is Memory.
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.