tools/eval: add +overlay feature

Now, inline evaluation will display results in an overlay next to the
cursor, rather than in the minibuffer (unless it gets too big, in which
case it'll use a popup buffer).

init.example.el

@@ -76,7 +76,7 @@
        ;;docker
        ;;editorconfig      ; let someone else argue about tabs vs spaces
        ;;ein               ; tame Jupyter notebooks with emacs
-       eval              ; run code, run (also, repls)
+       (eval +overlay)     ; run code, run (also, repls)
        flycheck          ; tasing you for every semicolon you forget
        ;;flyspell          ; tasing you for misspelling mispelling
        ;;gist              ; interacting with github gists