Cache scalar regexes

This improves performance on the test.yaml attached to #158 by about
35% on my machine (0.39s -> 0.25s), as measured by
`time build/util/parse < test.yaml > /dev/null`.

src/exp.h

@@ -165,6 +165,15 @@ inline const RegEx& EndScalarInFlow() {
   return e;
 }
 
+inline const RegEx& ScanScalarEndInFlow() {
+  static const RegEx e = (EndScalarInFlow() || (BlankOrBreak() + Comment()));
+  return e;
+}
+
+inline const RegEx& ScanScalarEnd() {
+  static const RegEx e = EndScalar() || (BlankOrBreak() + Comment());
+  return e;
+}
 inline const RegEx& EscSingleQuote() {
   static const RegEx e = RegEx("\'\'");
   return e;

src/scantoken.cpp

@@ -297,8 +297,8 @@ void Scanner::ScanPlainScalar() {
 
   // set up the scanning parameters
   ScanScalarParams params;
-  params.end = (InFlowContext() ? Exp::EndScalarInFlow() : Exp::EndScalar()) ||
-               (Exp::BlankOrBreak() + Exp::Comment());
+  params.end =
+      (InFlowContext() ? Exp::ScanScalarEndInFlow() : Exp::ScanScalarEnd());
   params.eatEnd = false;
   params.indent = (InFlowContext() ? 0 : GetTopIndent() + 1);
   params.fold = FOLD_FLOW;