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.

Using the real unpickler is quite costly for now.
My measurements can be seen [here](https://gist.github.com/eregon/148999525abb5121e66f).
In summary:
* to compile Hello World: more than 10s instead of less than 1s and 1GB of RSS instead of 20MB.
* to compile testBigInt.oz: about 3 times slower (5 -> 15s, 12 -> 38s) and 4 times memory usage: 1GB instead of 256MB.

Of course, memory usage can be reduced with `--max-memory` and it does not impacts a lot the execution time but it means patching `bin/ozc`.
I am wondering if there is a not a leak, is it expected that the compiler would need to allocate more than 900MB of data structures just to compile a Hello World?