AgRest own full-featured expression parser

[stariy95]

The last part that is still require Cayenne is complex expressions. To fully decouple everything we should implement own full-featured parser. It still could be based on the Cayenne grammar for the max compatibility.

There's already a working prototype with Cayenne backend switched to the new Expressions in the 5.x-expressions branch. This implementation deprecates old code in the io.agrest.exp package and introduces io.agrest.exp.parser.

Grammar file is here: https://github.com/agrestio/agrest/blob/5.x-expressions/agrest-engine/src/main/jjtree/io/agrest/exp/parser/AgExpressionParser.jjt

[andrus]

5.x-expressions branch looks great. The approach is the best I can think of. Once we merge it, we can decide how much of the deprecated API we need to keep. For now a few comments:

• We need parser tests for all possible expressions
• (a micro-optimization) AND and OR operations should be "flattened" when appending more clauses kind of like CompositeExp.and and CompositeExp.or do.
• Wonder about the best way to document the supported grammar. We should no longer rely on Cayenne docs for this, and should provide a full reference in the Agrest docs (this should be a separate task and should not block the runtime expressions implementation).