Renamed symtable.Symbol to symtable.Key.

Makes more sense.
Updated comments + renamed fields.

Trying to make things more clear.
More comments.
Set up simplelang to produce an output file.

Simplelang will produce an output assembly file when run on a pl0 input file.
I think I'm done with the improvements I'll make to this compiler. Time to build a bigger language
on this foundation.