`const` qualifiers in s2n-bignum header file

Currently the s2n-bignum header files do not eagerly attach the `const` qualifier to buffer arguments, but this causes users to introduce const casts.
The customized version of AWS-LC has const already attached: https://github.com/aws/aws-lc/blob/main/third_party/s2n-bignum/include/s2n-bignum_aws-lc.h

Probably we can automatize generation of the function signatures from specifications.