[Tachyon-list] Shell working under boostrap

I tried to run the shell boostrapped and ran into a "callee is not a function" error... So I had the idea of adding source code locations to those error messages (why didn't I think of that before!). I found out we were missing the String.prototype.substr library function (not to be confused with its cousin String.prototype.substring).

The said function is now implemented, and the shell works under the bootstrap :)

- Maxime