In 5 extension traits definitions use "extend" to remove boilerplate, and drop one now-unneded trait

Using a macro makes this extension trait pattern easier and less repetitive.

There are five libraries on crates.io for this:

• extend and ext: Both reasonable candidates. I tried out both in a personal project and settled on extend (because it handled a case I wanted).
• extension-trait has very sparse docs.
• extension-traits and ext-trait seem to be by the same person? The usage isn't as good.

diffstat on .rs files of the extension trait commits is 5 files changed, 25 insertions(+), 56 deletions(-)