create field specific rules

[eviltester]

Currently the IF x THEN rules are constraints for row generation

Add support for field level rules and amendments e.g.

[fieldname].rules{

if[other field]>47 THEN "use this value";

}

[fieldname].rules{
this = this + "-xxx";
}

etc.

the {} block should be parsed as statements and we are string to create a DSL so we need to have a better parser.

a literal after THEN would be an implied return from the statement block

without an implied return or an actual "return X" - return the value of 'this' and set [fieldname] to 'this'

constraints would apply after the main generation and after the rules.