cxxrtl: escape colon in variable names in VCD writer.

cxxrtl: escape colon in variable names in VCD writer.

The following VCD file crashes GTKWave's VCD loader:

    $var wire 1 ! x:1 $end
    $enddefinitions $end

In practice, a colon can be a part of a variable name that is
translated from a Verilog function, something like:

    update$func$.../hdl/hazard3_csr.v:350$2534.$result