[mypyc] Initial optimization for f-string through a str.join() specializer

[97littleleaf11]

Description

This pull request adds a specializer for faster constructing f-string, which is translated into str.join() previously in mypy AST.

Test Plan

IR building tests:

• {var}
• empty string
• literal string
• unsupported method

Speed up:

running str_format_fstring
..........
interpreted: 0.295028s (avg of 5 iterations; stdev 2.3%)
compiled:    0.188493s (avg of 5 iterations; stdev 1.7%)

compiled is 1.565x faster

on this case:

@benchmark
 def str_format_fstring() -> None:
     a = []
     tmp_str = "Foobar"
     for i in range(1000):
         for _ in range(5):
             a.append(f'{tmp_str}')
         a.append(f'{tmp_str}-{i}')
         a.append(f'{i} {i*2.0} str')

     n = 0
     for i in range(100):
         for s in a:
             n += len(f"foobar {s} stuff")
             ss = f"foobar {s} stuff"
             n += len(f"{i}-{s}-{ss}")
     assert n == 36897500, n

Current master branch is 0.380x.

[JukkaL]

This is a great performance improvement! Left some comments. Maybe also incorporate the int-to-str improvements from the other PR?

[JukkaL]

At some point it should be possible to share some of the code above with the str.format implementation. No need to do anything about this in this PR.