In other words, support:
`disallowed_methods = ["alloc::vec::Vec::new"]` (a free function) in
addition to
`disallowed_methods = ["alloc::vec::Vec::leak"]` (a method).
Improve the documentation to clarify that users must specify the full
qualified path for each disallowed function, which can be confusing for
reexports. Include an example `clippy.toml`.
Simplify the actual lint pass so we can reuse `utils::fn_def_id`.
