Added CHAR and REAL data type.

For REAL datatype there is still some work to do:

- Need to handle all automatic upgrades int => real.
- Need to implement comparison operators for REALS
  and also mixed for INTs and REALs (see previous point).
Added comparison operators for REALs.
CONSTANTs now can be used in expressions.
Comparison operators work for CHARs and BOOLEANs.
Clean up.
Added logical operators AND and OR.
Added constants TRUE and FALSE.
Added BUGS file.