Motivation

I wanted to use musli with garde for field validation, however due to coherence its not possible to add blanket impl for Valid and Unvalidated and I would like to avoid requiring newtyping them.

Content

All this PR does is

impl<TValid, M> Encode<M> for Valid<TValid>
where
    TValid: Validate + Encode<M>{...}
    
impl<'de, TValid, M, A> Decode<'de, M, A> for Unvalidated<TValid>
where
    TValid: Validate + Decode<'de, M, A>,
    A: Allocator {...}
    
impl<'de, TValid, M, A> Decode<'de, M, A> for Valid<TValid>
where
    TValid: Validate<Context: Default> + Decode<'de, M, A>,
    A: Allocator {...}

here Decode for Valid automatically also validates.

Other consideration

1. It would make sense to extend the macro with an option so that Decode is not impl for TValid but only for Unvalidated<TValid> and Valid<TValid> to make it impossible to accidentally directly decode a type skipping validation, but this would require changing the macro to be able to handle feature specific special cases.

2. It would also be possible to not impl Decode for Valid and always force manual calls to validate(). This is always the case for types that do not have a default Validation context. it may even be possible to link validation context to musli's context

Todos

1. In the branch for this commit I have not incremented crate version numbers. (done in fork's main branch)
2. no tests were added