Add support for Postgres encode and decode expressions

[dbrgn]

Postgres supports string encoding and decoding (to and from bytea columns):

https://www.postgresql.org/docs/9.5/static/functions-string.html#FUNCTIONS-STRING-OTHER

Examples:

SELECT encode(data, 'base64') FROM blobcache;
SELECT encode(pubkey, 'hex') FROM public_keys;

Supported formats: base64, hex, escape.

Would be great if these were usable from diesel.

[sgrif]

You can declare any function you'd like with the sql_function! macro. http://docs.diesel.rs/diesel/macro.sql_function.html