Assorted niche optimizations

[ilevkivskyi]

These are few random micro-optimizations (plus few correctness fixes I noticed in the process):

• De-serializing large callables/overloads is very slaw because of Enum.__call__()
• Large unions without literals were forced through make_simplified_union() in code path that supposed to handle literals
• Now that SomeType.__eq__() is called more often and we preserve more original types in expand_type() I added couple fast paths after gathering call stats there.

In total this gives ~0.5% on self-check, but makes loading some numeric libraries from cache up to 10% faster.

[github-actions]

According to mypy_primer, this change doesn't affect type check results on a corpus of open source code. ✅

[A5rocks]

Could mypyc maybe do an optimized version of the Enum thing? Surely there's other marginal uses of this pattern too...