[mypyc] feat: cache len for iterating over immutable types

[BobTheBuidler]

Currently, if a user has a tuple and uses it as the sequence input for a for loop, the tuple length is checked once at each iteration which, while necessary for some container types such as list and dictionaries, is not necessary for iterating over immutable types tuple, str, and bytes.

This PR modifies the codebase such that the length is only checked at the first iteration, and reused from there.

Also, in cases where a simple genexp is the input argument for a tuple, the length is checked one additional time before entering the iteration. In those cases, we don't even need a length check at the first iteration step, and can read the result of that first len call instead.

[BobTheBuidler]

Looks like my tests failed only on 32-bit python, because the generated IR uses 32-bit size numbers? Not sure what to do here, I don't think this failure is indicative of an issue in the PR.