Disable unstable_name_collisions warning

We rarely attach trait methods to stdlib types so this warning is unlikely to
be a true positive It is a false positive for the methods defined in future.rs.
It's not always obvious which method is selected when it's available in the
stdlib but I haven't seen a build failure yet.  So let's disable the warning.

In future we might be able suppress it per method, see Rust issue 48919.

fish-rust/src/lib.rs

@@ -1,6 +1,7 @@
 #![allow(non_camel_case_types)]
 #![allow(dead_code)]
 #![allow(non_upper_case_globals)]
+#![allow(unstable_name_collisions)]
 #![allow(clippy::bool_assert_comparison)]
 #![allow(clippy::box_default)]
 #![allow(clippy::collapsible_if)]