move constants files into constants/
move miscellaneous asm into engine/
updated install instructions
make clean is no longer required to compile
preprocess asm files individually

this fixes a lot of previous hacks

first off, rgbds requires that labels from includes be marked as globals.
instead, 3626ddeb stuffed includes into the parent file in the preprocessor.
this meant one huge file got preprocessed every time, adding an additional
ten seconds to compile time.

running the preprocessor once for each file would create too much overhead,
so a list is fed into prequeue.py, which then makes calls to preprocessor.py.

this paves the way for compiling source files separately some day.

next, compiling previously required `make clean` to be executed first.
f3340de6 touched main.asm to force a fresh compile instead. this behavior
has been reverted. now, `make all` will only attempt to recompile if a
source file has changed.

preprocessor.py has some marginal changes. prequeue.py is created to keep
the original functionality of preprocessor.py intact. so many files are
preprocessed on first compile (1951 as of this commit) that the prequeue
call has been hidden.

compile time is reduced to 15-30 seconds on first compile, and 5-10 seconds
subsequently. the majority of this time is spent in rgbasm.
make: don't recompile if it's not necessary

the preprocessor queue was preventing files from being recognized as out-of-date

this highlights similar errors with graphics targets, which need to be fixed anyway
gbz80disasm: data detection

the is_data flag never did anything. now it does

encountering the end of a branch with outstanding labels will read anything between said labels as data
gbz80disasm: detect data tables referenced in asm
gbz80disasm: local/global address conversion + fix find_label
gbz80disasm: fix data handling

get the scalpel out of there
gbz80disasm: just use lowercase labels
replace broken mibbit links with kiwiirc
install: get pip and be less vague about the base rom
install: elaborate on make targets
pre: restore separate_comment's behavior
give prequeue a reason to exist
install: less abrasive outro
Merge pull request #149 from yenatch/new-install

reformat install + fix source preprocessing
Merge pull request #148 from yenatch/master

Re-organizing some engine-related asm and constants.
don't mix tabs/whitespace