Will be much much easier to tests new core routines without having to
re-create their context first.

Also, extract parse.asm from core.asm

This will soon allow use to seek and tell on input, which is necessary
for a second pass which is necessary for forward symbol references.

This require making `blkSel` a bit more flexible. Rather than having one
global selected blkdev, each app can select its own, in its own memory
space.

Also, adjust SD card recipe. Straightforward initialization and read!

Makes for a more straightforward test.

Also, clarify the role of recipes.

Also, remove `parts/platforms`. It makes no sense. Only recipes make
sense.

This introduces `core.asm` which includes routines used by other parts.

Also, come up with a way to make parts play well together memory-wise.