Make serde_with schemars support extensible

Implementing JsonSchema directly on serde_with::Schema<T, TA> works fine
when within serde_with. Once you are outside the crate, however, the
orphan rules prevent you from adding impls of the form

impl JsonSchema for Schema<T, MyCustomType> { ... }

We would like users to be able to make that impl (or something
equivalent) so we need to find a way to make it work. We make this work
by adding a new layer of trait indirection.

This commit introduces an new JsonSchemaAs<T> trait, adds a blanket

impl<T, TA> JsonSchema for Schema<T, TA> where TA: JsonSchemaAs<T>

and then changes all the existing impls to be for JsonSchemaAs<T>.

Now, when a user wants to add json schema support for their own type
they can implement JsonSchemaAs<T> for their custom type and the orphan
rules will allow it.

As a bonus, this seems to have made the extraordinarily confusing
"reached recursion limit" errors that I have been encountering
disappear. Which is nice.

serde_with/src/lib.rs

@@ -331,7 +331,7 @@ pub mod json;
 mod key_value_map;
 pub mod rust;
 #[cfg(feature = "schemars_0_8")]
-mod schemars_0_8;
+pub mod schemars_0_8;
 pub mod ser;
 #[cfg(feature = "std")]
 mod serde_conv;