HStore support

Nobody has claimed this yet.

https://www.postgresql.org/docs/current/hstore.html

In reality, when creating a new database, you're more likely to define a `jsonb` column than hstore. However, if you already have an hstore field defined, we should have a way to pull that data out and map it to `Hash(String, String)`. Since hstore is a flat key/value pair, you'll only ever get `{} of String => String` from it. Maybe we could have a special type for this.

```crystal
table do
# Treated the same as Hash(String, String)
column data : HStore
end
```

The issue names no repository files or tests. Start with the PostgreSQL hstore documentation and inspect the repository's existing database type mappings; done means an hstore column can be declared and its flat key/value data maps to Hash(String, String).

Written by the indexing model from the issue text.