Use token tree in const_asset

Instead of our current `const_assert` we can use a macro token tree and
call through to `assert`. Doing so makes the macro more general.

internals/src/macros.rs

@@ -137,11 +137,12 @@ macro_rules! debug_from_display {
 }
 
 /// Asserts a boolean expression at compile time.
+// Copied from: https://internals.rust-lang.org/t/nicer-static-assertions/15986
 #[macro_export]
 macro_rules! const_assert {
-    ($x:expr) => {{
-        const _: [(); 0 - !$x as usize] = [];
-    }};
+    ($($tt:tt)*) => {
+        const _: () = assert!($($tt)*);
+    }
 }
 
 /// Derives `From<core::convert::Infallible>` for the given type.