*COMPLETELY* rewrite most of expression handling.

*COMPLETELY* rewrite most of expression handling.
Features:
- More complex memory expressions like [bx-(bx-di)] are now calculated through
  as valid.
- Much better simplification.
Internal changes:
- More flexible n-way tree (rather than old binary-only).
- Many new functions to simplify and deal with very complex expressions.

svn path=/trunk/yasm/; revision=294