Support enumeration types for partition columns

[jasonmp85]

This is similar to #79 except maybe more surprising that it doesn't Just Work™:

CREATE TYPE bug_status AS ENUM ('new', 'open', 'closed');
# CREATE TYPE
# Time: 6.044 ms

CREATE TABLE bugs (
    id integer,
    status bug_status
);
# CREATE TABLE
# Time: 3.599 ms

SELECT master_create_distributed_table('bugs', 'status');
# ERROR:  42883: could not identify a hash function for type bug_status
# DETAIL:  Partition column types must have a hash function defined to use hash partitioning.
# LOCATION:  master_create_distributed_table, create_shards.c:109
# Time: 3.202 ms

Kinda weird that enum types don't have a built-in hash function defined, given they're basically glorified integers.

[jasonmp85]

Strangely, while hash_array fails to hash an array of a custom composite type (could not identify a hash function for type…), it can hash an array of an enumerated type. So perhaps we should look at it to see whether we're looking up hash functions in the best way possible.

[onderkalaci]

@jasonmp85 So, this might be related to the patch applied to citus very recently (Composite Type bug fix (New version)).

So, I'll check this too, missed that while checking for #79 .

[jasonmp85]

Oh, I think I misled. I said that hash_array could find a function for composite types (#79), but I think I was confusing that with this (enumeration types). So we're on our own for composite types, but as I said above, it's kinda weird that we don't find the right function for an enumeration type.