Fix a few bugs:

• static compound assignment/shift had bugs (would allow bogus stuff)
• dynamic compound assignments are consistent with static case. This never worked ever, it would always promote. Now it works.

Improve some performance:

• dynamic xor was done in a slow way
• remove null guard from + operator when we have enough type info to know its invalid
• add megamorphic cache (from @uschindler !) so we have a legit performant fallback

Add lots of tests.

I tested the benchmark script by disabling some caches manually:

• inline cache: 0.45s
• megamorphic cache: 0.82s
• no caches: 3.65s

So Uwe's cache really helps the worst case scenario, where tons of types are seen. Its no longer trappy now :)