Support existing types with custom conversions

I'm trying to use fields of type std::time::Duration and initialize them with strings like "500 ms". It doesn't appear to be possible right now.

I converted my code to build.rs so I can explore all documented functions. The import_types! documentation is unclear and insufficient. Actually, there is no example of build.rs in the rustdoc documentation, but I found an example in the typify sources.

Only with_replacement lets me replace a generated type with the existing one (std::time::Duration). The documentation for with_conversion appears to promise that behavior, but a new type is always generated. Combinations of with_replacement and other with_* don't seem to work.

Unfortunately, with_replacement doesn't let me specify custom serde for the field. Look e.g. for #[serde(with = "humantime_serde")] at https://docs.rs/humantime-serde/1.1.1/humantime_serde/ - it's specified on the field, not on the whole struct.

I tried some things with x-rust-type without any success. In any case, I don't see it as a clean approach. It should be possible for the schema to be Rust-agnostic and still support readable strings for duration.

Likewise, I don't want to use numbers for duration. It's too easy to confuse seconds with milliseconds.

Currently supported formats ("uuid", "date", "ip" etc) all support conversion from string. The problem with std::time::Duration is that it's standard (i.e. many std functions use it) but cannot be constructed from a string. So a different mechanism is needed.

Start with the import_types! rustdoc and the build.rs example in the typify sources. Compare the documented behavior of with_conversion and with_replacement, including their combinations, and trace how generated fields receive serde attributes. Done should support an existing std::time::Duration field with custom string conversion without requiring schema-specific Rust types.