Removed the usage of `default` from the Option API

This implements parts of the changes to `Option` I proposed and discussed in this thread: https://mail.mozilla.org/pipermail/rust-dev/2013-November/006254.html, and on IRC.

This PR consists of two different changes to the same two functions:
- Renamed `map_default -> map_or` and `mutate_default -> mutate_or_set`.
- Swapped the parameters of both functions to reflect the order given by their new names.