Faster set creation with generator expression

[JukkaL]

Currently set(f(x) for x in it) is slower than {f(x) for x in it}, since the first one is evaluated like this:

set([f(x) for x in it])  # Note extra [...]!

We create a needless temporary list, which adds overhead.

It would be better to generate similar code for both alternatives.

For extra credit, do a similar optimization for other suitable candidates that are
handled by mypyc.irbuild.specialize.translate_safe_generator_call.

[rednayan]

want to do it as my first contribution to open source. Need some guidance.

[JukkaL]

Some more hints:

• Look at the IR generated for {f(x) for x in it} and set(f(x) for x in it) and look for differences (the IR is stored in build/ops.txt).
• The set(...) case is handled in translate_safe_generator_call (mypyc.irbuild.specialize). You'd need to add a new specialization for builtins.set.
• The set comprehension is handled in transform_set_comprehension (mypyc.irbuild.expression).