Added some notes about gmp.h and a devel directory to the doc directory.

doc/devel/gmp-h.txt

@@ -0,0 +1,37 @@
+Some compilers with hacks:
+====================================
+- GNUC
+- DEC C
+- SCO OpenUNIX 8 cc
+- __cplusplus
+
+Useful/undocumented macros in gmp.h:
+====================================
+__GMPN_ABS(x) - absolute value
+__GMPN_MAX(h, i) - max of h and i
+
+__GMPN_UINT_MAX - (int) ~0
+__GMPN_ULONG_MAX - (unsigned long) ~0
+__GMPN_USHRT_MAX - unsigned short) ~0
+
+__GMPN_LIKELY(cond) - branch likely hint
+__GMPN_UNLIKELY(cond) - branch unlikely hint
+  - e.g. if UNLIKELY(c >= 0L)....
+
+Note: the __GMPN_ macros are redefined with nicer names in gmp-impl.h
+
+mp_bits_per_limb - number of bits per limb
+gmp_version - GMP version number i.j or i.j.k as a null terminated string
+gmp_errno - integer corresponding to a GMP error condition:
+  0 none
+  1 unsupported argument
+  2 division by zero
+  4 sqrt of negative
+  8 invalid argument
+
+Undocumented mpn functions prototyped in gmp.h:
+===============================================
+mp_limb_t mpn_preinv_mod_1(mp_srcptr sp, mp_size_t sn, mp_limb_t x, mp_limb_t xinv) - 
+  compute {sp, sn} mod x given a precomputed inverse xinv of x
+
+