Duration cleanup and doctests

[kflynn]

This is best reviewed commit by commit.

• Pull the unneeded "use delegate::delegate" that didn't quite get removed before my previous PR merged
• Move Duration to gateway-api/src/duration.rs
• Switch to meaningful variable names in the TryFrom implementations
• Oh, yeah, run cargo fmt, that's a thing 🤦‍♂️
• Clean up some test comments and reorder test_gep2257_from_valid_duration cases to match test_gep2257_from_str
• Finally, do a monster doc hopefully-improvement and add doctests

[shaneutt]

Looks good, thank you!

One thing I noticed: we didn't actually enable your tests in CI, but we can't just yet because there's a problem with the generated code when it comes to doctests that I need to take some time and resolve (#49). That doesn't need to hold up this PR though, tests are passing when run manually and I'll get that sorted out shortly.