Work on improving Sail error messages

- Modified how sail type error messages are displayed. The
  typechecker, rather than immediately outputing a string has a
  datatype for error types, which are the pretty-printed using a
  PPrint pretty-printer. Needs more work for all the error messages.

- Error messages now attempt to highlight the part of the file where
  the error occurred, by printing the line the error is on and
  highlighting where the error message is in red. Again, this needs to
  be made more robust, especially when the error messages span
  multiple lines.

Other things

- Improved new parser and lexer. Made the lexer & parser handling of
  colons simpler and more intuitive.

- Added some more typechecking test cases

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