Fixes for compilation with static_assert and constexpr

I've also made some fixes in my  0.9.5-tweaks branch that might be of interest, although I'm not sure what's the best resolutions for these.

0c79766 Reverted from lessThan to < in step
error "no matching function for call to 'lessThan'"

3b30872 Switched from highp_int_t/highp_uint_t to int64/uint64 in integer functions
warning "shift count >= width of type"