Chainable require hooks

Changes for 0.015 - 2023-07-23

• [test] [bugfix] Add basic tests for all hooks, fix a couple of bugs (hooks previously not working).
• [doc] Various improvements to documentation, notably add documentation about hook ordering. Annotate code in Synopsis more. Remove mention of filter:: subnamespace and replace it with precheck:: and postcheck::.
• New hooks: debug::dump_source::{stderr,logger}.