Align on the caching design choice

[patacca]

We should enforce on the python API the same deisgn choices over caching/uncaching that we used on python-binexport/qbindiff.

The design choices can be summarized in the following points:

• Function objects are eagerly loaded in the constructor of Program and never deallocated
• BasicBlock objects are not cached by default. In order to access them in a cached way there are two ways: either with the context manager (with statement) or by manually calling Function.preload() and Function.unload() methods.
• Everything below basic block will be cached by default (Instruction, Operands, ...)