Macro attributes to specify From and Into trait types for structs and enums (#817)

* serde macro support for type conversions through From and Into trait

* Revisions requested by dtolnay

* Additional changes requested by dtolnay

test_suite/tests/compile-fail/type-attribute/type_attribute_fail_from.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Deserialize)] //~ ERROR: proc-macro derive panicked
+#[serde(from="")]
+enum TestOne {
+    Testing,
+    One,
+    Two,
+    Three,
+}

test_suite/tests/compile-fail/type-attribute/type_attribute_fail_into.rs

@@ -0,0 +1,11 @@
+#[macro_use]
+extern crate serde_derive;
+
+#[derive(Serialize)] //~ ERROR: proc-macro derive panicked
+#[serde(into="")]
+enum TestOne {
+    Testing,
+    One,
+    Two,
+    Three,
+}