Basic: variable renaming. Save 2 kbytes.

Basic: variable renaming. Save 2 kbytes.

Also, add variables.txt file with start of documenting meanings of
variables.

List of renamings/savings:

ZZ%   -> S%   : 131 bytes
ZL%   -> X    : 550 bytes
A%    -> A    : 192 bytes
E%    -> E    :  32 bytes
R%    -> R    : 381 bytes
AR%   -> AR   :  30 bytes
AY%   -> AY   :  71 bytes
AZ%   -> AZ   :  33 bytes
B%    -> B    :  47 bytes
AA%   -> AA   :  64 bytes
AB%   -> AB   :  25 bytes
F%    -> F    :  21 bytes
FF%   -> FF   :  14 bytes
ER%   -> ER   :  41 bytes
PR%   -> PR   :   7 bytes
T%    -> T    :  46 bytes
R0-9% -> R0-9 :  31 bytes
T0-9% -> T0-9 :  42 bytes
S1-4% -> S1-4 :  25 bytes
U0-9% -> U0-9 :  44 bytes
ZK%   -> ZK   :  10 bytes
ZI%   -> ZI   :  10 bytes
RC%   -> RC   :  16 bytes
K%/V% -> K/V  :  21 bytes
SD%   -> SD   :  16 bytes
ZS$   -> S$   :  40 bytes
HM%   -> H    :  10 bytes
SZ%   -> SZ   :  39 bytes
LV%   -> LV   :   9 bytes
EO%   -> O    :  18 bytes
C%    -> C    :   4 bytes
P%    -> P    :   4 bytes