Separate code loading from host library loading

`DefaultHost::load_library()` accepts a `Package` or `MastForest`. These forms load code but do not register event handlers. Execution later fails with `UnhandledEvent` when the code emits an event.

Keep `load_library()` for `HostLibrary`. Add separate methods for loading only a package or MAST forest. Add a test using a core precompile event.