Add @vectorize decorator

[certik]

In NumPy I think it's called @vectorize, in Fortran it's the elemental attribute. In ASR we have to mark that a given function/subroutine is "elemental" and if so, allow to call it on arrays.

Then add ASR pass that will transform this into explicit loops, so that the LLVM backend can compile it.

[Thirumalai-Shaktivel]

Is this issue fixed?
@certik @czgdp1807