Make tnetstring compatible with upcoming Rust version (>0.6)

The upcoming Rust version is for some parts of the Rust language not compatible with older versions:

 - Various keywords are dropped (move, pure, fail / assert macros).
 - Rename LinearMap -> HashMap.
 - LinearMap::find_ref() is dropped. I used HashMap::find() instead.
 - Fix implementation syntax (double colon is deprecated).
 -  Various core library function changes.

I compiled tnetstring successfully with this Rust version:
$ rustc -v
rustc 0.6 (2d28d64 2013-05-17 15:52:25 -0700)
host: x86_64-unknown-linux-gnu