cxxrtl: track aliases in VCD writer.
cxxrtl: track aliases in VCD writer.

This commit changes the VCD writer such that for all signals that
share `debug_item.curr`, it would only emit a single VCD identifier,
and sample the value once.

Commit 9b39c6f7 added redundancy to debug information by including
alias wires, and increased the size of VCD files proportionally; this
commit eliminates the redundancy from VCD files so that their size
is the same as before.

cliffordwolf/yosys,antmicro/yosys,cliffordwolf/yosys,cliffordwolf/yosys,SymbiFlow/yosys,cliffordwolf/yosys,cliffordwolf/yosys,SymbiFlow/yosys,YosysHQ/yosys,antmicro/yosys,antmicro/yosys,YosysHQ/yosys,cliffordwolf/yosys,antmicro/yosys,cliffordwolf/yosys,YosysHQ/yosys,cliffordwolf/yosys,YosysHQ/yosys,antmicro/yosys,SymbiFlow/yosys,YosysHQ/yosys,YosysHQ/yosys,antmicro/yosys,YosysHQ/yosys,SymbiFlow/yosys,SymbiFlow/yosys,SymbiFlow/yosys,YosysHQ/yosys,SymbiFlow/yosys,SymbiFlow/yosys,cliffordwolf/yosys,antmicro/yosys,antmicro/yosys