ApplyOnceRecording​(AngouriMath.​Entity,​AngouriMath.​Core.​Transformations.​RewriteRecording)

A method of its own, and that is the whole reason for it. The closure below
captures the rule set and the recording, and the compiler allocates the object
holding them where they come into scope -- so writing this inline in
ApplyOnce(AngouriMath.Entity) put one allocation on every rewrite in the
library whether or not anybody was recording. Measured: it cost `Simplify` a
fifth of its allocation on the benchmark expressions.