Internals: Optimize temporary memory allocations (#6517)

This patch gets rid of over 80% of temporary dynamic memory allocations
(when a malloced node is immediately freed with no other malloc in
between). It also gets rid of over 20% of all calls to malloc.

It's worth ~3% average verilation speed up with tcmalloc, and more
without tcmalloc.

src/V3String.cpp

@@ -327,10 +327,6 @@ string VString::replaceWord(const string& str, const string& from, const string&
     return result;
 }
 
-bool VString::startsWith(const string& str, const string& prefix) {
-    return str.rfind(prefix, 0) == 0;  // Faster than .find(_) == 0
-}
-
 bool VString::endsWith(const string& str, const string& suffix) {
     if (str.length() < suffix.length()) return false;
     return str.compare(str.length() - suffix.length(), suffix.length(), suffix) == 0;