Predicate Compiler for Evaluators

[omervk]

Predicates are currently created as ASTs, but are not compiled or optimized. Instead, evaluating them means traversing the tree in its entirety, something that, in a tight loop (like going over partitioning functions for all files in a table), is suboptimal.

Compiling the expressions into a minimal syntax tree that embeds the right-hand side values would let us optimize it ahead of time in several ways, including:

1. Compressing equals-or-equals trees to in.
2. Dropping always-true and always-false expressions entirely.

This issue therefore includes three things:

1. Representing expressions as a value-embedded AST.
2. Creating an optimizer framework.
3. Slowly building more and more optimizations.

This could probably be done using existing tools, without reinventing the wheel :)