PSL next_a / next_e support

[Blebowski]

Currently next_a and next_e operators are not supported. NVC fails with:

** Fatal: cannot handle PSL kind P_NEXT_A in build_node
    > ../src/psl_next_a.vhd:55
    |
 55 |   NEXT_0_a : assert always (a -> next_a[3 to 5] (b));
    |                                  ^^^^^^^^^^^^^^^^^^
make: *** [Makefile:16: psl_next_a] Error 1

Steps to reporduce:

git clone https://github.com/Blebowski/psl_with_ghdl
cd psl_with_ghdl
git checkout port-to-nvc
cd sim
make psl_next_a

or

make psl_next_e