Port format changes from Flatbush v3

[mourner]

The Flatbush storage format changed a bit — now there's an 8-byte header containing all additional info necessary for recreating the index.

Let's port the changes so that the libraries are interoperable.

https://github.com/mourner/flatbush

[logankaser]

I'm interested in porting the changes, is the new header format described somewhere?
I've been looking at mourner/flatbush@v2.0.4...v3.0.0, but it's not entirely clear to me what is new.

[logankaser]

Updated interpretation:

Flatbush Header:
(offsets in bytes)
0: Magic byte (0xfb)
1: VersionAndType
2-3: Node size, as a uint16
4-7: Number of items, as a uint32

VersionAndType:
(offsets in bits)
0-3: Verison
4-7: Type (type of coordinates / maps to typed array types in js impl)