User defined constraints Improvements

[philzook58]

User defined constraints can't refer to preassigned variables
We have a general provenance problem that occasionally bites us. The substituter eagerly destroys some information which is hard to recover.
Having said that, I don't know what extra constraints you could write that would usefully refer to preassigned variables.

A special code path to inject variables preassigned to constants could be done without too much difficulty be reaching back into the hvar preassignment config at param generation time

[philzook58]

It could be useful to retain C line number connection to operation id so that user can easily force certain operation orderings.