After thinking very hard, I am in favor of this. I would have liked to keep the mpn prefix, but it does make it harder work together with GMP and, as you say, adding the flint onto that does make it more clumsy.

I think I would like to abbreviate names in the mpn_extras module even more, just like GMP.

Examples:

flint_mpn_mulhigh_7 -> nn_mulhi_8
flint_mpn_sqrhigh_normalised_4 -> nn_sqrhi_4n
flint_mpn_mullow_8 -> nn_mullo_8

I liked having verbose names in the beginning, but now it becomes annoying with the long names. Although it is not necessary to abbreviate sqrhigh to sqrhi, it is useful for editing and viewing when names are equally long. And this is very low-level, so it will probably not affect end-users.

Fine with me.