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/V3String.cpp

@@ -109,6 +109,15 @@ string VString::spaceUnprintable(const string& str) {
     return out;
 }
 
+string VString::removeWhitespace(const string& str) {
+    string out;
+    out.reserve(str.size());
+    for (string::const_iterator pos = str.begin(); pos != str.end(); ++pos) {
+        if (!isspace(*pos)) out += *pos;
+    }
+    return out;
+}
+
 bool VString::isWhitespace(const string& str) {
     for (string::const_iterator pos = str.begin(); pos != str.end(); ++pos) {
         if (!isspace(*pos)) return false;