clean up assembly parsing
start laying groundwork for improved arithmetic commands
more work on common arithmetic commands
remove SETZERO instruction; it's identical to (SET @foo, 0), so no point making life complicated
add INVPOW and ROOT instructions; no idea if these actually do the relevant calculations or not
add some documentation on assembly syntax
document INVPOW/ROOT
SUBSTR seems to be taking a length, document this
tidy up some missing arguments in disassembly output
fix line numbers in compare