Support for ROMs whose size isn't a multiple of 1024 bytes

[michalderkacz]

This new code generates identical UF2 files as the original code for ROMs whose size is a multiple of 1024 bytes.

It allows the size of the last chunk in the unique_chunks to be less than 1024.

It generates additional UF2 blocks for the "240p test suite" ROM but unfortunately this ROM still doesn't work.

[michalderkacz]

Just for reference. Here is the simplified implementation in Go that avoids the intermediate flattening.