[xmlsec] Mscrypto IS patch

[Wouter]

> Attached is the implementation of the scheme outlined bellow. I have
> placed code in xmlsec-core
> so other xmlsec-crypto libraries can re-use it. Also I am going to
> remove src/mscrypto/bignum.c
> and include/xmlsec/mscrypto/bignum.h soon because there is nothing left
> in these files :)

What happened with xmlSecMSCryptoNodeGetBigNumValue and
xmlSecMSCryptoNodeSetBigNumValue ?

Wouter