Fixes to work with the new TLS api.
Fixes to work with the new TLS api.

TLS keys are no longer functions but static variables.
TLS-related functions are no longer unsafe =)

tomaka/glfw-rs,jhasse/glfw-rs,mneumann/glfw-rs,brson/glfw-rs,PistonDevelopers/glfw-rs,erickt/glfw-rs,rlane/glfw-rs,larsbergstrom/glfw-rs,nstoddard/glfw-rs,servo/glfw-rs,bvssvni/glfw-rs,bjz/glfw-rs,derekdreery/glfw-rs,derekdreery/glfw-rs