Fix #123: box.index.count and box.index.iterator do not check arguments

This patch adds lua_checkfield function - a wrapper for lua_tofield that raises
an error if Lua type can not be converted to lua_field structure. The function
should be used instead of lua_tofield in all bindings where custom types are not
supported.
Fix #122: box.pack('p', t) segfault on recursive tables

This patch limits maximum recursion depth for luaL_packsize and luaL_packvalue
functions to 16 nested tables. This check is needed to prevent a crash
on recursive Lua tables.
Merge branch 'gh-123-box-count-arg-check' into stable