Place a copy of a string into the specified position C<pos> on the stack.
The value is stored in a new mortal SV.
+=for apidoc Am|void|XST_mUV|int pos|UV uv
+Place an unsigned integer into the specified position C<pos> on the stack. The
+value is stored in a new mortal SV.
+
=for apidoc Am|void|XST_mNO|int pos
Place C<&PL_sv_no> into the specified position C<pos> on the
stack.