dtasm should output error if push/pop is used as wrong operand

[orlof]

dtasm should give loud error if stupid programmer writes code like
set a, push
or
set pop, a

...coming from stupid programmer who just spent 3 hours without realizing this

[patflick]

unfortunately that is not soo easy, the lexer already turns POP and PUSH into the same opcode, and the lexer has no contextual information. i will think about this for a bit, but i don't see a nice solution to warn the user/programmer