Add support for a generate scope to vvp and the vpi routines.

Instead of just translating a generate scope to a named begin/end scope
this patch creates a generate specific scope (vpiScopeGenerate) that is
of the vpiGenScope type. This may not match the standard 100%, but does
allow the FST dumper to denote generate scopes differently than the
other scope types. Most of the VPI code treats a vpiGenScope just like a
named block so only the FST dumper should have different behavior.