[mypyc] feat: use get_expr_length_value in translate_len

[BobTheBuidler]

Currently, translate_len can determine the length of an RTuple at compile time

This PR extends this capability to all expressions supported by get_expr_length_value

This will constant fold the code example displayed in mypyc/mypyc#1152 without waiting for the implementation of #19886 which has a few steps ahead of it

Before:

def extend_and_specialize(items: list[dict[str, Any]]) -> None:
    types: Final[dict[str, Any]] = {i["name"]: i for i in items}
    types.update(
        {
            k[len("https://w3id.org/cwl/salad#") :]: v
            for k, v in types.items()
            if k.startswith("https://w3id.org/cwl/salad#")
        }
    )

After:

def extend_and_specialize(items: list[dict[str, Any]]) -> None:
    types: Final[dict[str, Any]] = {i["name"]: i for i in items}
    types.update(
        {
            k[27 : ]: v
            for k, v in types.items()
            if k.startswith("https://w3id.org/cwl/salad#")
        }
    )

Even after #19986, this will also work for some situations that constant folding cannot handle but get_expr_length can

[BobTheBuidler]

It would be nice to figure out how to avoid loading the constant at all, but I couldn't figure it out when I was implementing this PR

[JukkaL]

We can add an optimization pass at some point that removes unused pure values.

[JukkaL]

Thanks for the PR! Looks good.

[JukkaL]

We can add an optimization pass at some point that removes unused pure values.