Cleanups

Important fixes:
* Robust parsing of integers from JSON when we are right at the "edge" of the representable range. 
* Improve `Base58::Alphabet::from_char` 

Misc:
Enable additional automatic checks in glibc when compiling debug builds with GCC.
Just an oddity.  When I view Base58.h using the github web-based diff, then the indentation looks great.  If I then view the latest version of Base58.h on the github web interface (by clicking view) then I lose most of the indentation.  Not sure what would cause that.  Curious.