schemars::gen module name from 0.8.x is incompatible with 2024 edition

[woody77]

The 2024 edition now reserves gen as a keyword: https://doc.rust-lang.org/edition-guide/rust-2024/gen-keyword.html

This makes it difficult to use schemars from crates switching to the 2024 edition.

error: expected identifier, found reserved keyword `gen`
  --> ../../src/lib/assembly/util/src/paths.rs:63:16
   |
63 | fn path_schema(gen: &mut schemars::gen::SchemaGenerator) -> schemars::schema::Schema {
   |                ^^^ expected identifier, found reserved keyword
   |
help: escape `gen` to use it as an identifier
   |
63 | fn path_schema(r#gen: &mut schemars::gen::SchemaGenerator) -> schemars::schema::Schema {
   |                ++

[woody77]

It looks like some escaping needs to be added to the derive macro here:
https://github.com/GREsau/schemars/blob/v0/schemars_derive/src/lib.rs#L71-L77

[SimonSapin]

#376 and v0.8.22 changed the code emitted by proc macros to use r#gen raw identifier syntax so that it’s now possible (even if not pretty) to do the same in user code and use the 2024 edition. cargo --fix --edition can help make that change automatically.