Write simplified expressions to the cache file

Add code to create the simplified expression for a parsed attribute, and if it is non null write the ecl out to a cache file.

This should also contain a couple of options

• parse the generated ecl and compare it with the simplified expression to check it is identical.  (Will ensure that simplified expressions can always be read from the cache file.)
• Return the simplified expression instead of the parsed expression.  (May speed up syntax checking, and will ensure that simplified expressions do not cause other syntax problems.)

It may be helpful to set a flag (e.g., ob_simplified) on the defined symbol to allow simplified expressions to be identified.