extapi: Add memory_decompress_from_func

[adiee5]

This exposes a functionality that already was available to the internal kernal code. This function works in a similar manner to memory_decompress, but instead of taking an address of compressed data in r0, it takes, in r4, an address of a function that streams LZSA2 data byte by byte each call.

Rationale:

This was already available in the kernal and such a function might be useful in many scenarios, such as:

• decompressing data incoming from a serial connection on the fly
• decompressing stuff stored on a cartridge (usage attested in the kernal itself)
• decompressing data from the banked ram (and automatically handle the bank switching, obviously)

I'll write the proper documentation of the function once it gets greenlit.

[mooinglemur]

looks good to me, thanks. Seems like we should ultimately rename the function in the kernal too

[adiee5]

i'll write the docs after thursday

[adiee5]

it's written btw X16Community/x16-docs#242