Insert string Ry, into string Rx at position Rz
and load into Rw. If position is greater than the length of Rx, then
insstr: will append the string to the end of Rx.
Parameters:
Rw: A register that will
contain the modified string.
Rx: A register of the string to modify.
Ry | string: A string register
or constant of the string to insert.
Rz | position: A numeric
register or constant specifying the insert position in Rx.