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.

Look at the DefaultHost::load_library method and its current handling of Package and MastForest. Identify where event handlers are registered. Create separate methods for loading only a package or MAST forest without registration. Write a test that uses a core precompile event to verify the new behavior.

由索引模型根据 Issue 内容生成。