Print summary notes at end of assembly process.

The assembler now tells the user that the assembly process has
completed, how many bytes of machine code were generated, how many
errors were encountered, and then finally dumps the symbols.

Other updates in this commit include:
* Added tests for the > and < modifiers for immediate values to extract
  upper or lower 8-bit values from 16-bit expressions.
* Ignore the SAV directive if any errors were encountered.
* To display the number of errors encountered, code had to be added to
  track the error count.