feat(macros): support transformation of a function for #[dynify]

[loichyan]

Support using #[dynify] on a standalone function:

#[dynify::dynify]
async fn my_func(id: usize) -> String {
    todo!()
}
// It expands to
fn dyn_my_func<'dynify>(
    id: usize,
) -> dynify::Fn!(usize => dyn 'dynify + Future<Output = String>) {
    dynify::from_fn!(my_func, id)
}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.37%. Comparing base (3f2b478) to head (e94f2eb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #13      +/-   ##
==========================================
+ Coverage   98.19%   98.37%   +0.17%     
==========================================
  Files          12       12              
  Lines         830      859      +29     
==========================================
+ Hits          815      845      +30     
+ Misses         15       14       -1     

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