This allows us to get rid of the zasm.sh wrapper.

Also, make parseHexadecimal "tail" (HL). Soon, all routines in lib/parse
will do that, making the life of lib/expr easier.

Also, increase STDIO_BUFSIZE to 0x40 so that those while loops work.

That's my mega-commit you've all been waiting for.

The code for the shell share more routines with userspace apps than with kernel
units, because, well, its behavior is that of a userspace app, not a device
driver.

This created a weird situation with libraries and jump tables. Some routine
belonging to the `kernel/` directory felt weird there.

And then comes `apps/basic`, which will likely share even more code with the
shell. I was seeing myself creating huge jump tables to reuse code from the
shell. It didn't feel right.

Moreover, we'll probably want basic-like apps to optionnally replace the shell.

So here I am with this huge change in the project structure. I didn't test all
recipes on hardware yet, I will do later. I might have broken some...

But now, the structure feels better and the line between what belongs to
`kernel` and what belongs to `apps` feels clearer.

Instead of expecting a `USER_CODE` symbol to be set, we expect `.org` to be
set in all userspace glue code. This gives us more flexibility with regards to
how we manage that.

Moreover, instead of making `USER_RAMSTART` mandatory, we make it default to
the end of the binary, which is adequate in a majority of cases.

Will be useful for my upcoming mega-commit... :)

Most of register fiddling routines (which is now the only thing contained
in care.asm) are used by almost all userspace apps, often in inner loops.

That makes the penalty of using jump tables for those a bit too high.
Moreover, it burdens jump tables needlessly.

Because this unit is very small (now that string routines are out), it makes
sense to always include it in binaries.

And made them shorter in name. The new ascii.h file allow reuse in userspace
code.

We use zasm's ability to use labels in .equ directive.

We didn't do it before because for a while, we were in between scas
and zasm (scas was used in automated tests) so we needed to use the
lowest common denominator: zasm doesn't have macros and scas can't
use labels in .equ directives.

This forced us to add this layer of indirection. But now that we are
completely cut from scas' dependency, we can use this nice zasm's
ability.

It was a bad idea and having sms/pad interact directly with sms/vdp
is much simpler.

ref #64

The goal is to avoid mixing those routines with "character devices"
(acia, vpd, kbd) which aren't block devices and have routines that
have different expectations.

This is a first step to fixing #64.

Found via `codespell -q 3 -S ./tools -L splitted`

scas, it's not needed any more.

Otherwise, we overflow into the RAM mirror, which means we
overwrite the beginning of the RAM.

Yup, that's ultimately why I've just made this whole big zasm
refactoring in the previous commits. To allow for this.

But also, zasm is in much better shape now...

Forward reference of SHELL_MEM_PTR wasn't working (nor should it
have been...).

Now this is getting eminently exciting!