Two different types often get named `parts`; rename one of them.

[paulstansifer]

parts is a commonly-used variable name (or variable name suffix) for both EnvMBEs and for LazyWalkResess. This is confusing! We should use it only to refer to EnvMBEs, and change all occurrences of parts referring to LazyWalkReses to something else. Doesn't really matter what. Maybe lwrs. Maybe walker.

(Also, LazyWalkReses is kinda ... a big pile of stuff. Maybe it should be refactored apart somehow, but it seems to work pretty well as a unit, so maybe not.)

[paulstansifer]

(also, walk_ctxt is sometimes used for LazyWalkResess, but I think that is probably confuseable with context_elt)