Make try_from_target_usize method public

There is now no way to create a TyConst from an integer, so I propose making this method public unless there was a reason for keeping it otherwise.

compiler/stable_mir/src/ty.rs

@@ -122,7 +122,7 @@ pub fn kind(&self) -> &TyConstKind {
     }
 
     /// Creates an interned usize constant.
-    fn try_from_target_usize(val: u64) -> Result<Self, Error> {
+    pub fn try_from_target_usize(val: u64) -> Result<Self, Error> {
         with(|cx| cx.try_new_ty_const_uint(val.into(), UintTy::Usize))
     }