Features to support BTrees

`@heaven2` on Discord [asked about parsing BTrees](https://discord.com/channels/1246168296248578128/1291859740766502983/1292487560999993356), and in particular suggested that custom length fields (#1289) would help make this more ergonomic.

We haven't been able to find a description of the format that's available online, but `@heaven2` described the format roughly as having a header field `num_slots: u16` which indicates the number of slots followed by `num_slots` slots, each of which is a `u16`.

For a more detailed description, see the *Slotted Pages* section of *Database Internals* by Alex Petrov (page 52).