This patch adds some abstract enums to pass to the trace decl* APIs, so
the VCD/FST specific code can be kept in verilated_{vcd,fst}_*.cc, and
removed from V3Emit*. It also reworks the generation of the trace init
functions (those that call 'decl*' for the signals) such that the scope
hierarchy is traversed precisely once during initialization, which
simplifies the FST writer. This later change also has the side effect of
fixing tracing of nested interfaces when traced via an interface
reference - see the change in the expected t_interface_ref_trace - which
previously were missed.
The FST trace timescale used to be set in the constructor via
set_time_unit, but at that point we haven't normally opened the
file yet so it was just dropped. On top of that, we actually want
to use set_time_resolution... FST trace timescales now match the VCD.
Geza Lore
Wilson Snyder
Jamie Iles
Àlex Torregrosa
Todd Strader