Simplified the interpreter loop by moving the logic of unrolling the stack
of blocks into the compiler. The compiler emits now explicit instructions
for adjusting the stack of values and calling the cleaning up code for
:keyword:`break`, :keyword:`continue` and :keyword:`return`.

Removed opcodes :opcode:`BREAK_LOOP`, :opcode:`CONTINUE_LOOP`,
:opcode:`SETUP_LOOP` and :opcode:`SETUP_EXCEPT`. Added new opcodes
:opcode:`ROT_FOUR`, :opcode:`BEGIN_FINALLY` and :opcode:`CALL_FINALLY` and
:opcode:`POP_FINALLY`. Changed the behavior of :opcode:`END_FINALLY` and
:opcode:`WITH_CLEANUP_START`.
