Optimize tuple_tostring and tuple to yaml encoding

[rtsisyk]

Current implementation unpacks entire tuple to Lua and then calls yaml.encode on it.
It was the simplest solution I've found for support MsgPack tuples.

    /*
     * The method does next things:
     * 1. Calls :unpack
     * 2. Serializes the result using yaml
     * 3. Strips start and end of yaml document symbols
     */

    /* unpack */
    lbox_tuple_totable(L);

    /* serialize */
    lua_replace(L, 1);
    yamlL_encode(L);

    /* strip yaml tags */
    size_t len;
    const char *str = lua_tolstring(L, -1, &len);
    assert(strlen(str) == len);
    const char *s = index(str, '[');
    const char *e = rindex(str, ']');
    assert(s != NULL && e != NULL && s + 1 <= e);
    lua_pushlstring(L, s, e - s + 1);
    return 1;

Please use libyaml directly and "remove the PHP" (c) Kostja from the code!

See also #30

[rtsisyk]

Needed for @zlobspb
Solution is to use mp_snprint() function from libmsgpuck.

[rtsisyk]

1.7.4-177-g0a7ca4b2e