aml: Do not require unstable features from rustc that are not used

`type_ascription` and `box_syntax` are not required to compile the crate (and
`box_syntax` has been removed in any case).

aml/src/lib.rs

@@ -35,7 +35,7 @@
 //! combinator patterns to express the parse.
 
 #![no_std]
-#![feature(decl_macro, type_ascription, box_syntax)]
+#![feature(decl_macro)]
 
 extern crate alloc;