API for prime fields in Montgomery representation

In Montgomery representation, an element \(x\) is represented by \(xR \mod p\), where \(R\) is a power of 2 corresponding to the number of machine-words that can contain \(p\).

Encoding and decoding between classical and Montgomery representation is implemented by the following functions:

    void mpfq_TAG_mgy_enc(mpfq_TAG_dst_field, mpfq_TAG_dst_elt, mpfq_TAG_src_elt);
    void mpfq_TAG_mgy_dec(mpfq_TAG_dst_field, mpfq_TAG_dst_elt, mpfq_TAG_src_elt);