This commit reimplements the C# API in terms of a Wasmtime linker.
It removes the custom binding implementation that was based on reflection in
favor of the linker's implementation.
This should make the C# API a little closer to the Rust API.
The `Engine` and `Store` types have been hidden behind the `Host` type which is
responsible for hosting WebAssembly module instances.
Documentation and tests have been updated.
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.