Support multiple trait bounds on generics and where clause

[taiki-e]

Workaround: https://github.com/taiki-e/pin-project-lite/issues/2#issuecomment-745190950

Currently, the following code does not be supported:

generics:

• [x] ? trait bounds: done in #9

pin_project! { 
    pub struct Maybe<'a, T: ?Sized> {  
        field: &'a mut T, 
    } 
} 

• [ ] multiple trait bounds:

pin_project! { 
    pub struct Multiple<'a, T: core::fmt::Debug + core::fmt::Display> { 
        field: &'a mut T, 
    } 
} 

where clause:

• [x] ? trait bounds: done in #22

pin_project! { 
    pub struct Maybe<'a, T> 
    where
        T: ?Sized,
    { 
        field: &'a mut T, 
    } 
} 

• [ ] multiple trait bounds:

pin_project! { 
    pub struct Multiple<'a, T> 
    where
        T: core::fmt::Debug + core::fmt::Display
    { 
        field: &'a mut T, 
    } 
} 

reference: https://doc.rust-lang.org/reference/trait-bounds.html#trait-and-lifetime-bounds

[taiki-e]

This is a bug, but maybe a limitation of declarative macros, and I'm not sure if it can be fully fixed.

[taiki-e]

? trait bounds are supported on both generics and where clause. (#9, #22)

[taiki-e]

Workaround: This can be avoided by moving bounds to where clause and splitting it.

  pin_project! { 
-     pub struct Multiple<'a, T: core::fmt::Debug + core::fmt::Display> { 
+     pub struct Multiple<'a, T: core::fmt::Debug> 
+     where
+        T: core::fmt::Display,
      { 
          field: &'a mut T, 
      } 
  }

  pin_project! { 
      pub struct Multiple<'a, T> 
      where
-         T: core::fmt::Debug + core::fmt::Display
+         T: core::fmt::Debug,
+         T: core::fmt::Display,
      { 
          field: &'a mut T, 
      } 
  }