This change adds a new function, BN_bn2bin_padded, that attempts, as
much as possible, to serialise a BIGNUM in constant time.
This is used to avoid some timing leaks in RSA decryption.
Initial fork from f2d678e6e89b6508147086610e985d4e8416e867 (1.0.2 beta).
(This change contains substantial changes from the original and
effectively starts a new history.)
