This changes pointer calculations in size comparions to a form that
ensures that no out-of-bound pointers are computed, because even their
computation yields undefined behavior.
Also, this changes size comparions to a form that ensures that neither
the left-hand side nor the right-hand side can overflow.
Identifiers starting with an underscore and followed immediately by a capital letter are reserved by the C++ standard.
The only header guards not fixed are those in the headers auto-generated from java.
These functions are intended for compatibility with legacy software,
and are not normally needed in new secp256k1 applications.
They also do not obeying any particular standard (and likely cannot
without without undermining their compatibility), and so are a
better fit for contrib.
Tim Ruffing
Dan Raviv
Pieter Wuille
Andrew Poelstra
Gregory Maxwell