Simple character save on +=1 and -=1

[Woccz]

Pending #48

Don't think it should be hard to implement.

Thank you.

[dbaumgarten]

This is only safe to to, if the compiler is absolutely certain that a is not a string.
Because for strings ++ is not then same as +=.

Therefore #48 would need to be done first

[Woccz]

Noted.

An equivalent but uncommon optimisation could also be implemented at the same time for strings:
x+=" " → x++.