Implement the ivl_do_not_elide property
Only get the vpiSysTfCall in getsimtime.c when needed.
Imlement vpi_handle(vpiScope, 0) for NULL handles

This is not part of the standard, but it convenient, and may be
used by the libveriuser to implement some bits of PLI1.
Implement vpi_handle(vpiLeft/RightRange, ...) for signals.

This returns a constant object that can be treated like an
expression, so following the letter of the LRM.