avoid future Option::reduce name conflict

[ahl]

Per a recent clippy warning

warning: a method with this name may be added to the standard library in the future
   --> progenitor-impl/src/to_schema.rs:257:18
    |
257 |                 .reduce(),
    |                  ^^^^^^
    |
    = warning: once this associated item is added to the standard library, the ambiguity may cause an error or change in behavior!
    = note: for more information, see issue #48919 <https://github.com/rust-lang/rust/issues/48919>
    = help: call with fully qualified syntax `to_schema::OptionReduce::reduce(...)` to keep using the current method