Use self.ischema_names so that it's modifiable

apparently, at least tests dependend on this variable being
modifiable, so use the one in self, not the one defined in the file
above
fdbsql does not support check constraints
Added todo

I'm pretty sure that doubel and numeric are the same, and builtins
are better than custom (also our DOUBLE doesn't print)
Revert "Set referred schema even if default"

This reverts commit 4a1397364a615e496054f1a586e83ee13ffef2f0.
This may have fixed that test, but it broke a whole bunch of others
especially in the reflection section
Copied is_disconnect from Postgres psycopg dialect

This is necessary for the right error to be thrown up
We don't support two phase transactions
We don't support row triggers
We don't support order by labels
throw error on lengthless varchar

like firebird
Switched to TEXT always having None length

FDBSQL doesn't let you set the TEXT length, and always returns
a length of 65535.
Added a couple keywords
Updated list of reserved keywords

these were taken from SQLGrammar.jj:15092
Copied in DefaultRequirements

The default ones are not included with pip, so I just copied the
contents in. Needs a whole bunch of cleanup.
Added temporary_tables requirement
Made FOR UPDATE a no-op

This is similar to other ORMS
We support the different constraint options

specifically deferrablity, match, and cascade
Support cascade, not deferrable_fks

Well, we do support deferrable_fks, but the tests are messed up