Change = to : in define_derive_adhoc

Having discussed this and thought about it, I still want to change this, even though Nick doesn't agree.

(Prompted by point 2 in #69 (comment 3029178), although I've wanted to change this for a while.)

Todo list

1. make d-d accept : !362 (merged)
2. document : as primary !362 (merged)
3. update guide examples !375 (merged)
4. update reference examples !405 (merged)
5. update arti.git tpo/core/arti!2209 (merged)
6. update @nickm's experimental serde and builder branches nickm/serde-deftly!1 nickm/build-deftly!1
7. bump "template export semver" minimum (since tests can't cope otherwise) !429 (merged) !430 (merged)
8. update all tests !431 (merged)
9. remove support for = !431 (merged)