Change the name of wit-bindgen's host implementation traits. #5890

Instead of naming the host implementation trait something like `wasi_filesystem::WasiFilesystem`, name it `wasi_filesystem::Host`, and avoid using the identifier `Host` in other places. This fixes a collision when generating bindings for the current wasi-clock API, which contains an interface `wall-clock` which contains a type `wall-clock`, which created a naming collision on the name `WallClock`.