rust: init: consolidate init macros

Merges the implementations of `try_init!` and `try_pin_init!`. These two
macros are very similar, but use different traits. The new macro
`__init_internal!` that is now the implementation for both takes these
traits as parameters.

This change does not affect any users, as no public API has been
changed, but it should simplify maintaining the init macros.

Reviewed-by: Björn Roy Baron <[email protected]>
Reviewed-by: Martin Rodriguez Reboredo <[email protected]>
Reviewed-by: Alice Ryhl <[email protected]>
Signed-off-by: Benno Lossin <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
[ Cleaned a couple trivial nits. ]
Signed-off-by: Miguel Ojeda <[email protected]>

Author: BennoLossin