Ban russian letter `с` in source code

[Gumix]

It can confuse e.g. if someone will grep for _fk_сonstraint and will get only one result.

$ git grep -i "с" src/
src/box/memtx_tree.cc:1064:  * Сmp_def in the functional index is the functional key and the
src/box/schema.cc:283:  /* _fk_сonstraint - foreign keys constraints. */
src/box/schema.cc:291:  /* _ck_сonstraint - check constraints. */
src/box/sql/build.c:2063:    * In case of <СREATE TABLE> and <ALTER TABLE ADD COLUMN>
src/lua/socket.c:516:   /* Сonvenient version: `events' is a string ('R', 'W', 'RW') */
src/lua/utils.h:205:* @brief Return CTypeID (FFI) of given СDATA type
src/lua/utils.h:207:* @param ctypename С type name as string (e.g. "struct request" or "uint32_t")