Use-site binder hygiene bug for `let`

I've previously discussed this with @mflatt , but I figure I should make a ticket.

Consider this program:

#lang rhombus/and_meta

block:
  let x = "outer"
  defn.macro 'm $id':
    'let $id = "inner"
     x'
  m x

It returns "inner", but I think it should return "outer" like the similar Racket program does:

(define x "outer")
(define-syntax-rule (m id)
 (let ([id "inner"])
   x))
(m x)

I want to point this out now because I saw https://github.com/racket/racket/pull/4929, which proposes to add something new to the syntax-local- zoo to support the implementation of Rhombus let. I'm a little skeptical of the current implementation strategy for let given that it gets this use-site binder hygiene wrong, and it feels like it'd be unfortunate to expose more low level syntax-local- operations that could point users towards implementations of scoping structures that don't get hygiene right.

In contrast, an implementation of let using definition contexts and local-expand should be able to avoid this problem because one of the things definition contexts encapsulate is a set of local use-site scopes. I don't know enough about Rhombus to know what other implementation constraints there are, though.

Start by reproducing the Rhombus example in the issue and compare its result with the similar Racket program. Read the current Rhombus let implementation and the definition-context and local-expand approach discussed in the report; done means the macro expansion preserves the outer x and returns "outer" without introducing an unsuitable low-level operation.