radicle-surf: rename serialize to serde

Instead of creating a named feature flag, opt for using the `serde`
package name as the feature flag name. This makes it more clear that
the feature is only enabling serde serialization and nothing else.

This patch also cleans up the testing flags by removing the serialize
flag for testing, as this should just always be tested, and also adds
the syntax flag for future testing.

Signed-off-by: Fintan Halpenny <fintan.halpenny@gmail.com>
X-Clacks-Overhead: GNU Terry Pratchett

test/Cargo.toml

@@ -33,7 +33,7 @@ features = ["test"]
 
 [dev-dependencies.radicle-surf-test]
 path = "../radicle-surf/t"
-features = ["serialize", "test"]
+features = ["test"]
 
 [dev-dependencies.git-storage-test]
 path = "../git-storage/t"