begin0 splicing

[jackfirth]

Resyntax ought to eliminate begin0 in the right-hand-side of define forms since those are always unnecessary. Saw this in benknoble/frosthaven-manager#106, where having this rule would have made another refactoring produce better output.

#lang resyntax/test

test: "begin0 in right hand side of define can be removed"
--------------------
#lang racket
(define (foo)
  (define x (begin0 42 (displayln "foo")))
  (* x 2))
--------------------
--------------------
#lang racket
(define (foo)
  (define x 42)
  (displayln "foo")
  (* x 2))
--------------------