Put functions in scope in the :config block

[jyp]

This fixes #1032, which also provides background.

With this change, use-package p puts in scope all the symbols declared by package p in scope of its :config block. The tradeoff is that init is slightly slower if interpreted. Compiling init.el gives the same runtime behavior as previously.

Perhaps there is a way to change use-package--eval-when-compile to obtain both fewer warnings at compile time and best runtime complexity, but this is beyond my elisp understanding.