Get global variables of taichi kernels and functions using __globals__

[lin-hitonami]

Concisely describe the proposed feature
We currently use inspect.getclosurevars to get global variables used in taichi kernels and functions. It can minimize the size of the global variable dict. However, it has many limitations. As mentioned in #3709, it ignores the variables in the annotations. It also ignores variables in listcomps and dictcomps.

Describe the solution you'd like (if any)
We can change back to the method in #282 to solve the problem.