[PATCH] Add crc32(text) & crc32(bytea)

Hi,

While answering one of the recent questions [1] I wanted to use
crc32(text) and discovered that it's missing out-of-the box. Of
course, one can use `substr(md5(x), 1, 8)` with almost the same effect
but it's less convenient and could be slower (I didn't do actual
benchmarks though). Also it's incompatible with third-party software
that may calculate crc32's and store the results in PostgreSQL.

I vaguely recall that I faced this problem before. Supporting crc32
was requested on the mailing list [2] and a number of workarounds
exist in PL/pgSQL [3][4]. Since there seems to be a demand and it
costs us nothing to maintain crc32() I suggest adding it.

The proposed patch exposes our internal crc32 implementation to the
user. I chose to return a hex string similarly to md5(). In my humble
experience this is most convenient in practical use. However if the
majority believes that the function should return a bigint (in order
to fit an unsigned int32) or a bytea (as SHA* functions do), I'm fine
with whatever consensus the community reaches.

[1]: https://www.postgresql.org/message-id/CAJ7c6TOurV4uA5Yz%3DaJ-ae4czL_zdFNqxbu47eyVrYFefrWoog%40mail.gmail.com
[2]: https://www.postgresql.org/message-id/flat/auto-000557707157%40umail.ru
[3]: https://stackoverflow.com/questions/28179335/crc32-function-with-pl-pgsql
[4]: https://gist.github.com/cuber/bcf0a3a96fc9a790d96d

--
Best regards,
Aleksander Alekseev