[mypyc] feat: optimize f-string building from Final values

[BobTheBuidler]

We can do some extra constant folding in cases like this:

from typing import Final

BASE_URL: Final = "https://example.com"
PORT: Final = 1234

def get_url(endpoint: str) -> str:
    return f"{BASE_URL}:{PORT}/{endpoint}"

which should generate the same C code as

def get_url(endpoint: str) -> str:
    return f"https://example.com:1234/{endpoint}"

This PR makes it so.

[BobTheBuidler]

This PR is now ready for review.

[BobTheBuidler]

This can probably be extended later on for non-final values that type check as Literals ex:

def test():
    abcd = "abcd"
    efgh = "efgh"
    if some_check():
        ijkl = "ijkl0"
        some_numbers = 123
        return f"{abcd}{efgh}{ijkl}{some_numbers}{some_fn()}"
    else:
        ijkl = "ijkl1"
        some_numbers = 456
        return f"{abcd}{efgh}{ijkl}{some_numbers}{some_fn()}"

But I think that goes beyond the scope of this PR. Once we merge this it will be easy to modify the literal helper fn.

[JukkaL]

Thanks for the PR! The approach looks good, just a few minor comments.

[BobTheBuidler]

Thanks for your feedbacks. Tests added, comment removed.