[mypyc] Specialize isinstance() with mixed and nested tuples

[rheard]

Fixes mypyc/mypyc#1227

This is just an idea for how to improve performance, for an issue that was exposed to me because of the issues in PR #22097 .

isinstance(x, (A, B)) is specialized when every class in the tuple is a primitive type (#19949) or every class is a native class (mypyc/mypyc#413). Anything else falls back to the generic path, which builds a new tuple and calls PyObject_IsInstance on every call, so the primitive types in the tuple lose their fast checks as well. That covers:

• a primitive type mixed with a native class: isinstance(x, (float, MyNativeClass))
• a primitive type mixed with any other class: isinstance(x, (float, Decimal))
• nested tuples, even when every class is a primitive type: isinstance(x, (int, (float, str)))

The chained checks added in #19949 could be generalized: flatten nested tuples, use the fast checks for primitive types and native classes, and check any other class with its own PyObject_IsInstance call, so no tuple is built. In a quick microbenchmark this takes isinstance(x, (float, MyNativeClass)) from about 50 ns to about 5 ns per call.

Relatedly, isinstance(f(), ()) currently returns False without calling f().

[ilevkivskyi]

Thanks for all the mypyc PRs! I will probably leave them all to @JukkaL and @p-sawicki (or maybe I will take one-two if they are busy).