auto merge of #8564 : alexcrichton/rust/ifmt+++, r=graydon

See discussion in #8489, but this selects option 3 by adding a `Default` trait to be implemented by various basic types.

Once this makes it into a snapshot I think it's about time to start overhauling all current use-cases of `fmt!` to move towards `ifmt!`. The goal is to replace `%X` with `{}` in 90% of situations, and this commit should enable that.

diff --cc src/libstd/fmt/mod.rs
Simple merge

diff --cc src/libstd/fmt/parse.rs
Simple merge