Switched to rust libcore

The code still requires cleanup, for now main.rs re-exports all
required std symbols.
Fixed travis build.
Fixed demo code using libstd.
Added missing lang items and cleaned up the reset.
Changed all 'use std::' to 'use core::'.
Added todo note for std::cmp.
Merge pull request #35 from farcaller/back-to-core

Switch to rust libcore.