bump to v0.5.6

Merge pull request pgcentralfoundation#808 from tcdi/0.5.5-prep

Release pgx 0.5.4 (pgcentralfoundation#802)

This release covers an incorrect handling of some Datum arguments,
some missed SpinLock macro bindings that we wanted to add to the C shim,
and introduces a base API for dynamic background worker registration!

Release pgx 0.5.3 (pgcentralfoundation#787)

This cuts pgx 0.5.3, which is mostly a bugfix release that covers a handful of
errors that had crept in to the end of the 0.5.0 test and release cycle.

Merge pull request pgcentralfoundation#770 from tcdi/0.5.2-quick

prepare for v0.5.2

prepare for a quick 0.5.1

- merge in pgcentralfoundation#763
- bump version numbers

Release pgx 0.5.0 (pgcentralfoundation#751)

Release 0.5.0-beta.1

Bump for v0.5.0-beta.0 (pgcentralfoundation#583)

* Bump for v0.5.0-rc.1

* ^H^H^H^Hbeta.0

Merge branch 'develop'