GRT: add ghdl_runfor, ghdl_rununtil and ghdl_restart

[umarcor]

In #14, an example was contributed for wrapping GHDL and executing the simulation step by step through __ghdl_simulation_step.

Further interesting examples would deal with simulation time explicitly. I am unsure whether a function exists for getting the simulation time explicitly after returning from __ghdl_simulation_step.

Ref ghdl/ghdl#1053.