[mypyc] Detect more issues when generating annotated HTML

[JukkaL]

Add new heuristics to detect various potential performance issues when using -a foo.html.
The issues include various generic (non-specialized) operations, uses of functools and
itertools, and slow isinstance checks that use runtime-checkable protocols.

Implement a mypy AST visitor that is used to detect some issues that would be harder to detect
when analyzing the generated IR.

Support annotation priorities so that if multiple annotations are generated for a line, only the
highest-priority ones are shown. This is a bit crude but useful, since often multiple heuristics
are triggered by some inefficient code, and duplicate annotations would be verbose and
sometimes confusing.