Move BignumInt definitions into a header file.

Move BignumInt definitions into a header file.

This allows files other than sshbn.c to work with the primitives
necessary to build multi-word arithmetic functions satisfying all of
PuTTY's portability constraints.

(cherry picked from commit 2c60070aad2d959a9e7e850523352c23c6aa7009)

Cherry-picker's notes: required on this branch because it's a
dependency of f8b27925eee6a37df107a7cd2e718e997a52516e which we want.