Output nested tables last

The TOML format has a restriction that if a table itself contains
tables, all keys with non-table values must be emitted first. This led
to the following error:

$ echo '{"a": {}, "b": 1}' | rq -T
[ERROR] [rq] Encountered: TOML serialize error
[ERROR] [rq] Caused by: values must be emitted before tables

Fix this by using tables_last to output the tables after other keys.

This affects all output formats that serialize using serde. The ordering
is unimportant in the other formats, so that is harmless. The
performance impact should be small.

src/value/mod.rs

@@ -6,6 +6,7 @@ use serde_json;
 use std::collections;
 use std::fmt;
 use std::io;
+use toml::ser as toml_ser;
 
 pub mod avro;
 pub mod cbor;
@@ -159,7 +160,7 @@ impl serde::ser::Serialize for Value {
             Self::Bytes(ref v) => v.serialize(s),
 
             Self::Sequence(ref v) => v.serialize(s),
-            Self::Map(ref v) => v.serialize(s),
+            Self::Map(ref v) => toml_ser::tables_last(v, s),
         }
     }
 }