Use walrus operator for env-assignment syntax

[Technologicat]

Now that Python 3.8 has added the walrus operator :=, it would make more sense to override that (instead of left-shift <<) to indicate env-assignment in let, do, and similar constructs. It would also help make IDEs happy, because := is a name-binding operator also in standard Python.

The rules would remain the same they are now (as of v0.14.3 and v0.15.0): when a name := value operation is encountered, if the context allows an env-assignment, then it is an env-assignment; otherwise leave it unprocessed, delegating to the standard meaning of := (just like we now, somewhat less sensibly, delegate to the standard meaning of <<).

Doing this will remove the need for noqa: F821 for names declared in let and do expressions, as well as allow the Lispy dialect (variant of Lispython) to work without triggering any undefined-name errors in static code analyzers for standard Python, such as flake8.

We could add name := value now as an alternative syntax, and deprecate the old syntax name << value once Python 3.8 becomes the minimum supported version. See #1 for the language version support schedule.

Note to self: To get started on this, grep the codebase for envassign and LShift. These are already handled centrally, so the required changes should be rather minor. We have a version-sensitive wrapper for NamedExpr in unpythonic.syntax.astcompat that will simply not match anything in 3.6 and 3.7.