Add methods to directly get value(s) specific types
Add methods to directly get value(s) specific types

Now `attrs.convert_into::<(T, U)>()` can be used instead of
`<(T, U)>::from_attributes(&mut attrs)`, and
`attr.convert_into::<T>()` can be used instead of
`T::from_attributes(attr)`.

They may help us simplify code because they don't require explicit
type specification and `use AttributeValue(s)`.

l1048576/fbxcel