[BUG] Single-character labels

[Simon Glover]

 At the moment, the assembler doesn't seem to like single character 
 labels. For instance, this code:

       bsr aa
       print "Didn't branch \n"
       end
  aa:  print "Branched \n"
       end  

 prints "Branched", as one would expect, but this code:

       bsr a  
       print "Didn't branch \n"
       end
  a:   print "Branched \n"
       end

 provokes a bunch of error messages from the assembler:

  constantize_integer(): Called with non-numeric argument 'a'! at
Parrot/Assembler.pm line 1320
        Parrot::Assembler::constantize_integer('a') called at
Parrot/Assembler.pm line 1188
        Parrot::Assembler::handle_arguments('bsr a', 'bsr_ic', 'a') called
at Parrot/Assembler.pm line 622
        Parrot::Assembler::process_program_lines() called at ./assemble.pl
line 14

 Now, before I go ahead and try to fix this, I thought I better ask
 whether there's any particular reason that single-character labels
 shouldn't be allowed? 

 Simon