Add compilation behavior for Eval

To make `eval` performance feasible on a per-pixel scale, we need to have some sort of "compilation" behavior. This would involve parsing an expression, matching the necessary Ops to complete that expression, and then storing the "compiled" result for re-use.

The performant-critical goal here is to eliminate the need to re-match Ops when re-running an equivalent expression (but this must allow for numerical variables to change, e.g. value at current pixel position)

The issue names Eval, expression parsing, Op matching, and compiled-result reuse, but no files or tests. Start at the Eval entry point and trace how expressions are parsed and Ops are matched. Done means equivalent expressions avoid repeated Op matching while numerical variables can still change per pixel.

Written by the indexing model from the issue text.