[Lang] More evaluative build_for

[windwhiterain]

When build ast for "for loop", decorators like "static", "grouped", exprs like "ndrange" are detected directly from ast. However, "mesh-for" is detected via evaluating the iterable statement's value. I made some change that using similar evaluation to detect "grouped" and "ndrange" so that end user can wrap these method into there own iterable. I'm motivated when developing a type hint rich wrapper for taichi, during which I found no way to wrap "grouped" nor "ndrange" into typed one.

[CLAassistant]

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[feisuzhu]

/rebase