This adopts a "writer" concept, vaguely inspired by the ta parameter
writer. This might turn out to be not a great idea if the
response/offsets for heterogenous commands are too inconvenient to
keep track of.
This breaks every example that uses maple--only
example/maple_controller is updated to use the new interface.
This combines my iso9660 parsing code, with all of the prior gdrom packet
interface / command code.
The example, on real Dreamcast hardware, displays the first 2048 bytes [1] of every
file in the root directory on the serial console.
[1] or the size of the file, whichever is smaller
