 small depends on exception.h

Reported by @alyapunov 

There are a pair of functions with 'xc' postfix in small sub-module. That why "exeption.h" is included and the small depends on exceptions definition.

This dependency hinders usage of small submodule as a standalone project.

I beleive 'xc' functions must be moved to somewhere like 'small_xc.h' (and possibly 'small_xc.cc')
