[mypyc] Modify specializers and add int.__pow__ in test-data/fixture

[97littleleaf11]

Description

• Move simple tuple and list creation to new functions. Each specializer only focus on one situation now.
• Add int.__pow__ in test-data/fixture

Test Plan

This change should pass all existing tests.

[97littleleaf11]

The following test case leads to a test failure:

source_e = [0, 1, 2]
e = list((x ** 2) for x in (y + 2 for y in source_e))
assert e == [4, 9, 16]

Got this when testing

Unsupported left operand type for ** ("int")

However I cannot reproduce this by direct compilation.

[TH3CHARLie]

my guess is that you need to modify the fixture you are using

[JukkaL]

Thanks for the PR! Looks good. It's easier to understand specializers if each of them does one thing only.