- Add HUSH_PRIVATE_IMPLEMENTATION guard to hush.h separating implementation from declarations
- Create hush_impl.cpp to compile all hush_*.h implementation as a separate translation unit
- Create hush_nSPV_impl.cpp to compile all hush_nSPV_*.h headers separately
- main.cpp now includes declarations only, no longer a unity build for hush code
- Add both new compilation units to Makefile.am
