Support IEEE time units and time precisions, #234. (#2253)

Includes `timescale, $printtimescale, $timeformat.
VL_TIME_MULTIPLIER, VL_TIME_PRECISION, VL_TIME_UNIT have been removed
and the time precision must now match the SystemC time precision.
To get closer behavior to older versions, use e.g. --timescale-override
"1ps/1ps".

src/V3Const.cpp

@@ -2067,6 +2067,10 @@ private:
                     case '%': break;  // %% - just output a %
                     case 'm': break;  // %m - auto insert "name"
                     case 'l': break;  // %l - auto insert "library"
+                    case 't':  // FALLTHRU
+                    case '^':  // %t/%^ - don't know $timeformat so can't constify
+                        if (argp) argp = argp->nextp();
+                        break;
                     default:  // Most operators, just move to next argument
                         if (argp) {
                             AstNode* nextp = argp->nextp();