Add timestamp built-in type
Relatively many formats store time  in some way; it would be useful to have a built-in type for the most common representations. Simple grepping by `formats` repository gives such files that propably uses UNIX timestamp, serialized as `u4`:

* bson.ksy
* fallout_dat.ksy
* gran_turismo_vol.ksy
* gzip.ksy
* lzh.ksy
* match_o.ksy
* openpgp_message.ksy
* phar_without_stub.ksy
* python_pic_27.ksy
* rtcp_payload.ksy
* rtp_packet.ksy
* uimage.ksy
* windows_evt_log.ksy
* windows_minidump.ksy
* zip.ksy

Generators should use appropriate target language type for timestamps