Simplify scanFollows usage (#510)

Use static functions to avoid declaring global vars and creating more
package init costs.  This change has no negative effects on benchmarks
in my testing.

scanner.go

@@ -2,26 +2,34 @@ package toml
 
 import "fmt"
 
-func scanFollows(pattern []byte) func(b []byte) bool {
-	return func(b []byte) bool {
-		if len(b) < len(pattern) {
-			return false
-		}
-		for i, c := range pattern {
-			if b[i] != c {
-				return false
-			}
-		}
-		return true
-	}
+func scanFollows(b []byte, pattern string) bool {
+	n := len(pattern)
+	return len(b) >= n && string(b[:n]) == pattern
 }
 
-var scanFollowsMultilineBasicStringDelimiter = scanFollows([]byte{'"', '"', '"'})
-var scanFollowsMultilineLiteralStringDelimiter = scanFollows([]byte{'\'', '\'', '\''})
-var scanFollowsTrue = scanFollows([]byte{'t', 'r', 'u', 'e'})
-var scanFollowsFalse = scanFollows([]byte{'f', 'a', 'l', 's', 'e'})
-var scanFollowsInf = scanFollows([]byte{'i', 'n', 'f'})
-var scanFollowsNan = scanFollows([]byte{'n', 'a', 'n'})
+func scanFollowsMultilineBasicStringDelimiter(b []byte) bool {
+	return scanFollows(b, `"""`)
+}
+
+func scanFollowsMultilineLiteralStringDelimiter(b []byte) bool {
+	return scanFollows(b, `'''`)
+}
+
+func scanFollowsTrue(b []byte) bool {
+	return scanFollows(b, `true`)
+}
+
+func scanFollowsFalse(b []byte) bool {
+	return scanFollows(b, `false`)
+}
+
+func scanFollowsInf(b []byte) bool {
+	return scanFollows(b, `inf`)
+}
+
+func scanFollowsNan(b []byte) bool {
+	return scanFollows(b, `nan`)
+}
 
 func scanUnquotedKey(b []byte) ([]byte, []byte, error) {
 	// unquoted-key = 1*( ALPHA / DIGIT / %x2D / %x5F ) ; A-Z / a-z / 0-9 / - / _