Deprecate lexically dependent behavior of Past/Rose/Fell/... without explicit domain= argument

[whitequark]

While the exact details of how Past/Rose/Fell/... are still subject to discussion in #526, it seems clear that the current behavior, with the domain taken (when not specified) from the lexically enclosing m.d.<domain> += statement, was a mistake. Let's deprecate it in 0.3 to be able to change it in 0.4.

[whitequark]

@awygle raised some good objections to this plan, and I now think that this deprecation would be too rushed to go well in 0.3.