Needed by #30.

Pretty major improvements to both of these, cpHLDE is now 5 bytes shorter and between 9 and 12 cycles faster due to branching, and writeHLinDE is now 2 bytes shorter and 21 cycles faster.

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

* Optimised intoXX functions

Rewrote intoXX functions to mainly rely on intoHL, as the HL instructions are smaller and faster. Also removed some redundant push and pop instructions. I edited the given unit tests to test these, and they seem to work as expected.

* Doesn't use self-modifying code

The number of bytes is the same as my previous attempt, with 11 more cycles in intoHL, so although I don't feel as clever this time it's still a good optimisation. I found an equivalent method for intoDE, however relying on intoHL still allows for  `ex (sp), hl` to be used in intoIX, which is smaller and faster.

* Update core.asm

* Tried harder to follow coding convention

Added tabs between mnemonics and operands, and replaced a new line I accidentally removed.

Fix misc. source comment typos

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

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.

scas, it's not needed any more.

This makes zasm suitable to run from ROM.

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...

This will allow interesting override scenarios, adding flexibility.

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

Instead of strings of variable length driving the iteration of the
registry, we do so through records that keep track of lengths and
counts.

Things are a bit more straightforward now.

This will allow me to make the ".org" treatment a bit less murky.

*sob*...

I'm about to split the global registry in two (labels and consts)
and the previous state of registry selection made things murky.
Now it's much better.

It allows to see much more easily where the action happens on the screen.

Also, remove zasm/test7 because it changes too much all time time
(whenever zasm changes) and isn't precise enough. Too much noise,
not worth it.