I'm pretty happy about how lightweight the implementation turns
out to be.

Also, remove needless argtype in CALL and RET

And move its doc to doc/asm.txt

Also, move doc to doc/asm.txt.

Also, fix the pcat recipe which was broken since the overlay change.
I hadn't noticed it because I didn't have to rebuild the MBR.

The idea is to consider assemblers as "runtime" apps instead of
placing them in the "bootstrap" section of the blocks. These apps
will be used for much more than bootstrapping.

Moved its documentation to doc/asm.txt and made its code blocks
more compact.