[mypyc] Add a special case for str_rprimitive in builder.builtin_len

[97littleleaf11]

Description

Closes mypyc/mypyc#835

• Add a branch for str_rpimitive in builtin_len
• Reduce redundant code
• Faster list/tuple built from str

Test Plan

• Add run tests for faster list/tuple built from str
• Modify one irbuild test. The original one is redundant.

[97littleleaf11]

I made a mistake last week that a cannot be assigned to Tuple[str...].

[97littleleaf11]

There is another testTupleBuiltFromList thus it's redundant.

[JukkaL]

Can you double check the perf impact by running some quick microbenchmark?

[97littleleaf11]

This branch:

interpreted: 0.000373s (avg of 2601 iterations; stdev 1.6%)
compiled:    0.000180s (avg of 2601 iterations; stdev 1.3%)

compiled is 2.070x faster

Master branch:

interpreted: 0.000357s (avg of 2675 iterations; stdev 4.6%)
compiled:    0.000233s (avg of 2675 iterations; stdev 2.2%)

compiled is 1.529x faster

Tested on:

def list_from_str() -> None:
    s1 = "aaaaa"
    n = 0
    for i in range(1000):
        list = [x for x in s1]
        n += len(list)
    assert n == 5000, n

[97littleleaf11]

Adding a wrapper function has little impact on performance, since the speed up is based on the optimized list comprehension:

interpreted: 0.000327s (avg of 2838 iterations; stdev 1.4%)
compiled:    0.000157s (avg of 2838 iterations; stdev 1.2%)

compiled is 2.084x faster

[JukkaL]

Looks good! Thanks for the updates.