mp_talloc: simplify a bit

Implement MP_GROW_ARRAY using MP_TARRAY_GROW. MP_GROW_ARRAY is basically
the earlier version of MP_TARRAY_GROW, and had different semantics. When
I added MP_TARRAY_GROW, I didn't dare to change it, but I think all code
that relied on the exact semantics of MP_GROW_ARRAY is gone now, or the
difference doesn't matter anyway. The semantic change is that now
(n+1)*2 elements are preallocated instead of n*2.

Also, implement MP_TARRAY_GROW using MP_RESIZE_ARRAY, which saves 1 line
of code.

In future, these macros should be part of TA.