Evaluate expressions in .rept
When fixing .unreq, don't print one instance immediately

When the .unreq is part of a .rept, both of them need to go along
with each other so that they end up at the right place in the repeated
output.
Explicitly check the $line variable in all conditions in parse_if_line

This avoids the hidden assumption that the parameter passed is
also available in $_.
Handle if conditions in .rept
Refactor handling of .set to a reusable function
Handle .set when expanding repetitions as well
Only replace symbols with their corresponding value if the symbol is defined

This avoids replacing a substring like x0123 in hex constants with
an empty string.