Speed up SIP build by indexing

[dvdkon]

I've been working on speeding up the build of QGIS' Python bindings and during that work, I've found a few places where SIP was needlessly slow.

The largest improvements are in places where SIP was doing some O(n) lookup in a loop. I've managed to speed up most of these by keeping a transparent index of various parts of Specification and then simply asking the index instead of looping over all items.

Some smaller improvements came from using identity comparison instead of equality and improving the name cache packer.

Together, my not-so-scientific tests on QGIS' bindings show a roughly 5x improvement in sip-build runtime (sans compilation).

[philthompson10]

Thanks for this. I won't be able to review it properly for a couple of weeks. Some initial stylistic feedback:

• I want to keep specification.py as clean as possible, so your extra tooling should go in a separate file.
• Every class and callable should have a docstring giving an informal description of its purpose written using proper English sentences.

[dvdkon]

Thanks, I've moved the IndexedLists to a separate file and documented them in hopefully enough detail.