Update default compilation flags to workaround a g++4.8 bug

NT2 fails to compile with g++-4.8, setting a special compiler option
fixes this without disturbing clang++...
Update core::slice implementation

Split the implementation between core::slice that uses none<long> and
core::normalized_slice that uses (more efficient) long

*Huge* performance improvements thanks to this commit!
Merge pull request #147 from serge-sans-paille/normalized-slice

Normalized slice