- NestingError derives from HTMLParseError, and is hence simplified.

- Thread the input position through all the code generation routines;
  all compile-time exceptions now possess lineno and offset.

- Restructured the code that inserts implied end-tags, and made it
  generate output that is the same as the input more often.  This was
  the hardest to get right, and I expect to be working more on it.