Advance class and function table pointers every compilation invocation

This is a small refactor to how the AT compilation step works.
Previously, each invocation of compile_file() would move the global
class and function table pointers to the end of Zend's respective
tables. This caused an insidious bug where blacklisted files would
occasionally insert whitelisted classes into Zend's class table,
causing havoc when later compiling subclasses of that class. Now,
we iterate over all new entries in Zend's class and function tables
upon each invocation of compile_file(). This causes negligible
performance overhead when there are only a few classes and functions
per PHP file, which is the typical case.
Compiler now sweeps for opcode arrays with multiple references

Upon encountering a whitelisted op_array that has multiple references
but does _not_ have an enforcement op_array, we now do something
intelligent depending on the situation (this previously sometimes
caused a crash). In this event, we check to see if our current
compilation sweep would encounter every other reference to the
op_array in question. If so, we can safely compile the doc annotation
and free the opcode array's corresponding literal array, because
we know that all other references will be later fixed up on this
sweep (this commonly happens when compiling files containing a
class hierarchy). If not, we throw a notice to the user and bail
before compilation so that we don't corrupt the other references.
Bump version to v0.6.3