feat: Add IntoOwned trait

[kdy1]

Closes #596

The signature should be

pub trait IntoOwned<'any> {
  type Owned: 'any;

  /// Make lifetime of `self` `'static`.
  fn into_owned(self) -> Self::Owned;
}

Description

• The caller determines the trait to call, so the lifetime 'any of IntoOwned<'any> is determined by the caller.

This is identical as fn into_owned<'x>(self) -> Self<'x>.

• The trait is implemented for nodes with any lifetime (not 'any), so it can be called if self is provided.

• The bound for generics is for<'aa> T: IntoOwned<'aa>, so the implementation of generic parameters do not matter.

[kdy1]

@devongovett I did it 😄

The code is not clean, though.

[kdy1]

Feel free to modify docs or codes 👍

[devongovett]

Thanks, this looks good. I'll play around with it in more detail tomorrow.