Fix toolchain setting in CI (#532)

* Fix toolchain setting in CI

Due to a misunderstanding of the actions-rs toolchain action, we were installing a toolchain but _not actually using it_!

* Update ci.yaml

set toolchain components explicitly

* Fixes to get code compiling on 1.51 again

* Update ci.yaml

Add rustdoc

* Update ci.yaml

switch to 1.52.1 for rustdoc links

* Update JSON fuzz target

* Fix missed refactoring in ci.yaml