Add endian conversions for scalar types

[bnert]

Based off this comment for PR: cycfi/artist#2 (comment)

Add functions to include/support.hpp:

• to_little_endian
• to_big_endian

for various scalar types (e.g. uint16_t, uint32_t, etc...).

[djowel]

Should be easy :-) Care for a PR? Some ideas here: https://github.com/boostorg/endian/blob/develop/include/boost/endian/detail/endian_reverse.hpp

Probably deserves to be a file on its own.

[bnert]

Sure! I'll most likely have some spare time this week to get something up.