Allow non-Serialize default values.

Default values that don't implement Serialize are now ignored, rather than causing a compile error.
This is done by simulating specialization using a technique copied from Rocket:
5ebefa97c9/core/lib/src/sentinel.rs (L391-L445)

Fixes #115

schemars_derive/src/metadata.rs

@@ -68,7 +68,7 @@ impl<'a> SchemaMetadata<'a> {
 
         if let Some(default) = &self.default {
             setters.push(quote! {
-                default: #default.and_then(|d| schemars::_serde_json::value::to_value(d).ok()),
+                default: #default.and_then(|d| schemars::_schemars_maybe_to_value!(d)),
             });
         }