Improve per-module compilation by checking modification time

[lukaszcz]

Currently, when checking whether a module should be recompiled we always compute the hash of the source and compare with the stored hash. We could also store the modification time, assume the source hasn't changed if the modification time didn't change and compute the source hash only if it did change. This would speed up stored module loading.

[lukaszcz]

No significant performance improvement is achieved by checking modification time. The ~1s loading time for compiled standard library is due to deserialization of the saved compiled modules, not due to hash computation.