lexer: fix ReadRune() to actually move the cursor

Signed-off-by: Alejandro Mery <amery@jpi.io>

lexer/reader.go

@@ -153,6 +153,9 @@ func (b *Reader) ReadRune() (rune, int, error) {
 
 	// decode rune
 	r, l := utf8.DecodeRune(b.buf[b.cursor:])
+	// step over
+	b.cursor += l
+
 	return r, l, nil
 }