Refactor `windowed` to use a backing array interface

[Raynos]

Currently windowed reads & renders from this.rows which is an array.

By changing that to an interface it can be swapped to by anything that's a synchronous API like C++ or lmdb

[heapwolf]

This is done.

[Raynos]

This is not done yet, the rows field is still public instead of private which does not allow it to be swapped easily without a breaking change.

[heapwolf]

Oh haha sorry my bad :)