Change the error type of the constructors of `Arc` and `UniqueArc` to be
`AllocError` instead of `Error`. This makes the API more clear as to
what can go wrong when calling `try_new` or its variants.
Signed-off-by: Benno Lossin <y86-dev@xxxxxxxxxxxxxx>
Reviewed-by: Andreas Hindborg <a.hindborg@xxxxxxxxxxx>
Cc: Gary Guo <gary@xxxxxxxxxxx>
Cc: Alice Ryhl <aliceryhl@xxxxxxxxxx>