Document pdex.bin file format and improve conversion script

[castholm]

I documented the pdex.bin file format. It is essentially a stripped-down ELF file consisting of metadata, a program segment and relocation entries.

I also rewrote pdex2elf.py, which did not actually work with modern pdex.bin files. I'm late to the Playdate game but I suspect (much) older versions of the SDK produced executable files in a much different format from what is in use currently.

The new pdex2elf.py script creates well-formed ELF file that can be analyzed by readelf or objdump and decompiled by Ghidra, and it can even be compiled back by pdc into a bit-for-bit idential copy of the original pdex.bin. I tested it with both some of my own WIP games as well as some non-encrypted games from the catalog and they round-tripped perfectly.

It is a PowerShell script which I expect to be a bit controversial 😛 but PS is the only shell scripting-like language I'm proficient in so take it for what it is. If you would like to port the functionality to Python or whatever you have my blessing. Update: I've now ported it to Python.

[castholm]

@jaames Pinging just in case you haven't yet seen this PR.

[gatesphere]

As of SDK 2.5.0, pdc now supports multi-segment pdex.elf files. Unknown if @castholm's script above will need any adjustments for this.

[castholm]

I believe that multi-segment fix is just so that the pdc compiler itself properly reads all segements in the input ELF file. It shouldn't have any effects on the pdex.bin format itself or the script's ability to round-trip a pdex.bin.

[castholm]

Ported the PowerShell script to Python.