Move OctalEscapeSequence to read_escape_char

This should simplify and improve implementation, make it easier to
implement template strings, and keep master a bit more in sync with
harmony.

Previous implementation wasn't broken, though the loop gave me the
impression it could read infinite numbers and annoyed me a bit. It was
also slightly unnecessary because the lookup involved only 3 characters.

test/compress/string-literal.js

@@ -0,0 +1,10 @@
+octal_escape_sequence: {
+    input: {
+        var boundaries = "\0\7\00\07\70\77\000\077\300\377";
+        var border_check = "\400\700\0000\3000";
+    }
+    expect: {
+        var boundaries = "\x00\x07\x00\x07\x38\x3f\x00\x3f\xc0\xff";
+        var border_check = "\x20\x30\x38\x30\x00\x30\xc0\x30";
+    }
+}