Fix ID type deserialization from signed integers (#1668)

The TOML specification defines integers as 64-bit signed integers and the
deserializer calls `Visitor::visit_i64`.

src/model/id.rs

@@ -251,6 +251,7 @@ id_u64! {
 /// }
 /// ```
 pub(crate) mod snowflake {
+    use std::convert::TryFrom;
     use std::fmt;
 
     use serde::de::{Error, MapAccess, Visitor};
@@ -274,6 +275,11 @@ pub(crate) mod snowflake {
             formatter.write_str("string or integer snowflake")
         }
 
+        // Called by formats like TOML.
+        fn visit_i64<E: Error>(self, value: i64) -> Result<Self::Value, E> {
+            u64::try_from(value).map_err(Error::custom)
+        }
+
         fn visit_u64<E: Error>(self, value: u64) -> Result<Self::Value, E> {
             Ok(value)
         }