This patch generates a compiler error if `host_fs` and `mem_fs` are
both enabled at compile-time. It also generates a compiler error if
none of them have been enabled.

`wasmer-wasi`'s default features don't change, except that it adds
`host_fs` in the default features, which maps to
`wasmer-vfs/host_fs`. It then ensures that the same behavior is
guaranteed.

We don't need to change how `wasmer-cli` and `wasmer-c-api` fetch
`wasmer-wasi` in this case.