Support multiple binary targets in `rust-run`

rust-run fails if cargo run needs a --bin argument, which is the case if:

1. The workspace has multiple binary targets.
2. No target is listed as the package's default-run

It is probably reasonably easy to support this. A possible interface update to rust-run could simply prompt for a target name iff both conditions above hold (multiple binary target, no default) OR it's called with an argument. The chosen target's name could be saved somewhere so further invocations of rust-run can just reuse it (unless called with an arg)

This is a quick and dirty way to get all binary targets from the current workspace, and the default target:

(let* ((json (with-temp-buffer
               (call-process "cargo" nil (current-buffer) nil
                             "metadata" "--format-version" "1")
               (goto-char (point-min))
               (json-parse-buffer)))
       ;; !! this is a bit dirty. !!
       (members (seq-map (lambda (wm)
                           (car (split-string wm " ")))
                         (gethash "workspace_members" json)))
       (targets
        (-flatten ; from dash.el, but not strictly required.
         (seq-map (lambda (v)
                    (seq-keep
                     (lambda (target)
                       (when (seq-contains-p
                              (gethash "kind" target) "bin" 'string=)
                         (gethash "name" target))) v))
                  (seq-map (lambda (package) (gethash "targets" package))
                           (seq-filter
                            (lambda (pkg) (seq-contains-p members (gethash "name" pkg)))
                            (gethash "packages" json))))))
       (default-run (gethash "default_run"
                             (car
                              (seq-filter
                               (lambda (item) (string= "raoc2021" (gethash "name" item)))
                               (gethash "packages" json))))))
  (message "Workspace members: %s" members)
  (message "Binary targets: %s" targets)
  (message "Default run target is: %s" default-run))

What do you think?

Start at the rust-run entry point and inspect how it invokes cargo run and handles arguments. Implement target selection for workspaces with multiple binaries, including reuse of a chosen target, and verify that rust-run works both with and without an explicit argument.