Completely encapsulate and separate PARI and FLINT interfaces.

* lib/cm-impl.h: Do not include PARI or FLINT headers.
  (cm_flint_mpzx_xplusa_pow_modmod, cm_flint_mpzx_gcd_mod):
  Remove function declarations.
* lib/cm.h (cm_pari_print_library, cm_flint_print_library,
  cm_flint_init, cm_flint_clear): Declare new functions.
* lib/pari.c: Include pari.h.
  (cm_pari_print_library): New function.
  (cm_pari_init): Call cm_flint_init.
  (cm_pari_clear): Call cm_flint_clear.
* lib/flint.c: Include FLINT headers.
  (cm_flint_mpzx_xplusa_pow_modmod, cm_flint_mpzx_gcd_mod):
  Make functions static.
  (cm_flint_print_library, cm_flint_init, cm_flint_clear): New function.
* src/params.h: Do not include pari.h.
* src/params.c (print_libraries): Call new functions.