Calculate litMatcher.want at codegen time (#95)

parseLitMatcher() was calling strconv.AppendQuote() each time, which my
profiler discovered was costing a measurably large amount of runtime
during parsing within dhall-golang.

This commit calculates litMatcher.want at codegen time so that we don't
have to compute the `want` value each time we parse with the litMatcher.

builder/builder.go

@@ -422,6 +422,11 @@ func (b *builder) writeLitMatcher(lit *ast.LitMatcher) {
 		b.writelnf("\tval: %q,", lit.Val)
 	}
 	b.writelnf("\tignoreCase: %t,", lit.IgnoreCase)
+	ignoreCaseFlag := ""
+	if lit.IgnoreCase {
+		ignoreCaseFlag = "i"
+	}
+	b.writelnf("\twant: %q,", strconv.Quote(lit.Val)+ignoreCaseFlag)
 	b.writelnf("},")
 }
 

builder/static_code.go

@@ -357,6 +357,7 @@ type litMatcher struct {
 	pos        position
 	val        string
 	ignoreCase bool
+	want       string
 }
 
 //{{ if .Nolint }} nolint: structcheck {{else}} ==template== {{ end }}
@@ -1245,25 +1246,20 @@ func (p *parser) parseLitMatcher(lit *litMatcher) (interface{}, bool) {
 	}
 
 	// {{ end }} ==template==
-	ignoreCase := ""
-	if lit.ignoreCase {
-		ignoreCase = "i"
-	}
-	val := string(strconv.AppendQuote([]byte{}, lit.val)) + ignoreCase // wrap 'lit.val' with double quotes
 	start := p.pt
 	for _, want := range lit.val {
 		cur := p.pt.rn
 		if lit.ignoreCase {
 			cur = unicode.ToLower(cur)
 		}
 		if cur != want {
-			p.failAt(false, start.position, val)
+			p.failAt(false, start.position, lit.want)
 			p.restore(start)
 			return nil, false
 		}
 		p.read()
 	}
-	p.failAt(true, start.position, val)
+	p.failAt(true, start.position, lit.want)
 	return p.sliceFrom(start), true
 }