The oldObtain linter, against stream-of-consciousness obtain

The oldObtain linter flags any occurrences of "stream-of-consciousness" obtain, i.e. uses of the obtain tactic which do not immediately provide a proof.

Example

There are six different kinds of obtain uses. In one example, they look like this.

theorem foo : True := by
  -- These cases are fine.
  obtain := trivial
  obtain h := trivial
  obtain : True := trivial
  obtain h : True := trivial
  -- These are linted against.
  obtain : True
  · trivial
  obtain h : True
  · trivial

We allow the first four (since an explicit proof is provided), but lint against the last two.

Why is this bad?

This is similar to removing all uses of Tactic.Replace and Tactic.Have from mathlib: in summary,

• this version is a Lean3-ism, which can be unlearned now
• the syntax obtain foo : type := proof is slightly shorter; particularly so when the first tactic of the proof is exact
• when using the old syntax as obtain foo : type; · proof, there is an intermediate state with multiple goals right before the focusing dot. This can be confusing. (This gets amplified with the in-flight "multiple goal linter", which seems generally desired --- for many reasons, including teachability. Granted, the linter could be tweaked to not lint in this case... but by now, the "old" syntax is not clearly better.)
• the old syntax could be slightly nicer when deferring goals: however, this is rare. In the 30 replacements of the last PR, this occurred twice. In both cases, the suffices tactic could also be used, as was in fact clearer.

opaque Mathlib.Linter.Style.linter.oldObtain : Lean.Option Bool

The oldObtain linter emits a warning upon uses of the "stream-of-consciousness" variants of the obtain tactic, i.e. with the proof postponed.