nodrop: Add nightly feature use_union

Use a separate crate (nodrop-union) so that we can conditionally use the
new union keyword without disrupting compatibility.

nodrop/Cargo.toml

@@ -22,6 +22,13 @@ std = ["odds/std"]
 # Use `needs_drop` to skip overwriting if not necessary
 use_needs_drop = []
 
+# Optional, nightly channel
+use_union = ["nodrop-union"]
+
 [dependencies.odds]
 version = "0.2.12"
 default-features = false
+
+[dependencies.nodrop-union]
+path = "../nodrop-union"
+optional = true