Add missing string escape sequences

lexer.go

diff --git a/lexer.go b/lexer.go
index c3a145c..91f93c9 100644
--- a/lexer.go
+++ b/lexer.go
@@ -332,6 +332,15 @@
 		} else if l.follow("\\n") {
 			l.pos += 1
 			growing_string += "\n"
+		} else if l.follow("\\b") {
+			l.pos += 1
+			growing_string += "\b"
+		} else if l.follow("\\f") {
+			l.pos += 1
+			growing_string += "\f"
+		} else if l.follow("\\/") {
+			l.pos += 1
+			growing_string += "/"
 		} else if l.follow("\\t") {
 			l.pos += 1
 			growing_string += "\t"