Revert "Replace the boot unpickler by the real one from Pickle.oz"

* It severely impacts compilation time (and uses a lot more memory).
* This reverts most of commit 2efe0be998664ff2dffa3a119efe42f1a70bf2e5 (See #141).
* The fix in Pickle.oz and the better comment are kept.
Merge pull request #153 from eregon/unpickle

Revert "Replace the boot unpickler by the real one from Pickle.oz"