`make update-mocks` is usually run in DEVELOPER mode, but then it includes
definitions for functions which aren't declared in non-DEVELOPER mode.
We hacked this in a few places, but it's fragile, and worst, now we
have EXPERIMENTAL_FEATURES as well, it's complex.
Instead, declare developer-only functions (but don't define them).
This is a bit more awkward if you accidentally use one in
non-DEVELOPER code (link error rather than compile error), but makes
autogenerating test mocks much easier.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
