Add support for displaying more information on failed assertion (not used yet)
Rearrange declarations and definitions in to_str_backend.hxx
Add note on compiler errors for missing abc::to_str_backend specializations
Rearrange definitions in to_str_backend.cxx

Mirror the changes in commit ca8f47e5a6838ce878988a53d7175eb2befacd23.
Add abc::to_str_backend specialization for character literals
Add abc::to_str_backend specialization for abc::pointer_iterator
Begin using new support for displaying more information on failed assertions
Fix GCC signed/unsigned warnings