Change lhs/inout parameter of assign functions to mutable ref

[9prady9]

This is required since by default a varialble reference immutable in
rust. However, arrayfire C API for assignment operations, af_assign_seq
and af_assign_gen, does use lhs as output. Thus, an Array passed as
lhs might get modifed some times, thus breaking the immutable ref rule.

This change fixes that by converting the lhs/inout paramters of
assignment functions to mutable references and removes return values
completely.

Fixes #222