Rename the FLINT .pxi files to .pxd files. Create a new file src/sage/libs/flint/types.pxd with just the type declarations (similar to GMP, very useful to avoid circular imports).

Component: misc

Author: R. Andrew Ohana, Jeroen Demeyer

Branch/Commit: b69be0d

Reviewer: William Stein, Jean-Pierre Flori

Issue created by migration from https://trac.sagemath.org/ticket/16428