prevent With clause name shadowing

Name shadowing can be quite a footgun. Example:

```
TutorialD (master/main): x:=with (x as true) x
("with type",Relation (attributesFromList []) (RelationTupleSet {asList = []}))
("with type",Relation (attributesFromList []) (RelationTupleSet {asList = []}))
TutorialD (master/main): :showexpr x
ERR: RelVarAlreadyDefinedError "x"
```

The original expression should almost certainly be rejected, but the relational expression validation is sound, so we would need to check that the new relvarname "x" is not mentioned in the with clause names.

I am toying with the idea of preventing name shadowing of relvars or other with clause macro names (even macros shadowing macros). It does seem that name shadowing can be sometimes useful when moving subexpressions around, but the risk and cost of mistakes here is very high.

Any objection?