[PATCH] Compression dictionaries for JSONB

Hi hackers,

This is a follow-up thread to `RFC: compression dictionaries for JSONB` [1]. I would like to share my current progress in order to get early feedback. The patch is currently in a draft state but implements the basic functionality. I did my best to account for all the great feedback I previously got from Alvaro and Matthias.

Usage example:

```
CREATE TYPE mydict AS DICTIONARY OF jsonb ('aaa', 'bbb');

SELECT '{"aaa":"bbb"}' ::  mydict;
     mydict
----------------
 {"aaa": "bbb"}

SELECT ('{"aaa":"bbb"}' ::  mydict) -> 'aaa';
 ?column?
----------
 "bbb"
```

Here `mydict` works as a transparent replacement for `jsonb`. However, its internal representation differs. The provided dictionary entries ('aaa', 'bbb') are stored in the new catalog table:

```
SELECT * FROM pg_dict;
  oid  | dicttypid | dictentry
-------+-----------+-----------
 39476 |     39475 | aaa
 39477 |     39475 | bbb

Current limitations (todo):
- ALTER TYPE is not implemented
- Tests and documentation are missing
- Autocomplete is missing

Future work (out of scope of this patch):
- Support types other than JSONB: TEXT, XML, etc
- Automatically updated dictionaries, e.g. during VACUUM
- Alternative compression algorithms. Note that this will not require any further changes in the catalog, only the values we write to pg_type and pg_cast will differ.

Open questions:
- Dictionary entries are currently stored as NameData, the same type that is used for enums. Are we OK with the accompanying limitations? Any alternative suggestions?
Your feedback is very much welcomed!

[1]: https://postgr.es/m/CAJ7c6TPx7N-bVw0dZ1ASCDQKZJHhBYkT6w4HV1LzfS%2BUUTUfmA%40mail.gmail.com

--
Best regards,
Aleksander Alekseev