Fix Constructor derive emitting redundant_field_names lint

[akundaz]

The Constructor derive for named-field structs was generating:

Self { field: field }

instead of the field init shorthand:

Self { field }

This triggers clippy::redundant_field_names on nightly Clippy when -D warnings is set, breaking crates that use #[deny(clippy::all)] or similar.

Fix: use #(#field_names),* in the struct literal rather than #(#field_names: #vars),*. Since the parameter names already match the field names, the shorthand is always valid here. This also removes a redundant variable alias and clone in struct_body.

[akundaz]

The CI failure (compile_fail) seems to be pre-existing and unrelated to this change. I can reproduce it locally on master without my commit applied — 4 of 136 compile_fail tests fail there too.