fixes #79

Again, same thing as in zasm.

Same thing as in zasm.

It can only print a decimal literal. But still, that's a big step because
I hadn't implemented decimal formatting yet.

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... :)

Also important for upcoming mega commit...

Will be important for a mega-commit I'm preparing.

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.

Can possibly avoid a lot of debugging pain.

Also, make empty strings be parsed as invalid by parseDecimal.

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

Let's see where it will lead us...

Because scas isn't used as a reference anymore, no need to blacklist its bugs
anymore.

There was a little bit of cruft.

This allow a lot more characters to fit on that tiny screen...

ref #41

There's a couple of bit fiddling instructions that didn't have their
IX/IY variant implemented yet and without this commit, each of them
would have required a special routine. Not anymore.

We now require less special handling.

That's a lot of code for such a small change, but there's a big difference
between 5 pixels and 4 pixels: 4 pixels requires compositing.

This was mostly lifted from my "tihello" example, but it required significant
adjustments.

This commit also introduces a font management system. A lot of fonts are
available online, but sources aren't always clear so to avoid copyright
landmines, I re-created my first 5x7 font from scratch.

As it is now, this resulting ROM gets "Collapse OS>" to be displayed on the
LCD screen. Much work still left to do.

ref #41

Most OS don't have python3 in /usr/bin/python...

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.

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.

Will help with #45

Needed by #30.

It needs bash after all. On OpenBSD, /usr/sh works, but not on Ubuntu.

* Fix for tools/zasm.sh being dependent on readlink -f (an issue on macOS, preventing builds)

* Wrap zasm.sh shebang in /usr/bin/env ; remove comment about BSDs

I've tested RAM usage when self-assembling and there weren't as high
as I thought. zasm's defaults now use less than 0x1800 bytes of RAM,
making it possible, theoretically for now, for a Sega Master System
to assemble Collapse OS from within itself.