Refactor HasSqlType to provide an array of compatible types.

* Intending to use in a new Row type to check types at runtime for
   dynamic queries and to guard against schema changes

 * Hoping the query! macro can utilize this to allow accepting N
   rust types for 1 sql type and returning N rust types for 1 sql
   type.

sqlx-core/src/executor.rs

@@ -64,7 +64,7 @@ pub trait Executor {
         Box::pin(async move { s.try_next().await?.ok_or(crate::Error::NotFound) })
     }
 
-    /// Analyze the SQL query and report the inferred bind parameter types and returned columns.
+    #[doc(hidden)]
     fn describe<'e, 'q: 'e>(
         &'e mut self,
         query: &'q str,