Define pointer offset type in coding conventions

--- code_conventions.txt        2014-04-01 03:48:29.000000000 +0400
+++ code_conventions-m.txt      2014-04-04 14:14:53.699583491 +0400
@@ -23,6 +23,7 @@
     | limb counts in multiprecision integers | mp_size_t                   |
     | limbs (unsigned/signed)                | mp_limb_t/ mp_limb_signed_t |
     | mp_limb_t arrays                       | mp_ptr/ mp_srcptr           |
+    | offset for an array                    | slong                       |
     | ui/ si function constants              | ulong/ slong                |
     | exponents (unsigned/signed)            | ulong/ slong                |
     | polynomial lengths                     | slong                       |