Feature request: Add ability to specify MDB_INTEGERKEY, MDB_INTEGERDUP and MDB_DUPFIXED flags

would it be possible for `txn:db_open(create, db?)` to get a few more parameters to enable using LMDB native integer keys?

something similar to this patch for the `py-lmdb` project:
https://github.com/jnwatson/py-lmdb/pull/94/commits/d3fc35a33b49c8fc1be36993cc2c57f6ed5a44fb

perhaps something like this:
```
txn:db_open(create, db?, integerkey?, integerdup?, dupfixed?)
```
with each of the new optional parameters defaulting to `false`

I would submit a pull request if it were not above my pay grade ;)

Start at the txn:db_open(create, db?) API and compare the requested behavior with the referenced py-lmdb patch. Trace how the existing optional arguments reach LMDB, then verify that integerkey, integerdup, and dupfixed default to false and enable the corresponding flags when set.

Written by the indexing model from the issue text.