[Lang] Build subscripted tuple assign

[windwhiterain]

Currently assigning to an subscripted list of tuples in taichi scope will results an error. For example:

a = [(0, 1)] * 3

a[1] = (0, 2) # results error

I modified build_basic_assign to support this.

I was motivated from my attempt to create custom iterable from custom data. I have to create a list of tuple like [(0, 1), (1, 3)] to fill in the ndrange.

[feisuzhu]

/rebase