std:: vs core:: namespace and other build errors

[mjt0k]

I'm trying to build this crate on debian, since btree is a new dependency of virtiofsd. But I'm having some.. issues with it.

$ cargo test --all-targets --no-default-features --features allimpls

This gives me numerous errors:

error[E0433]: failed to resolve: use of undeclared crate or module `std`
   --> src/impls/smallvec.rs:113:18
    |
113 |     type Iter<'a> = std::slice::Iter<'a, A::Item> where Self: 'a;
    |                     ^^^ use of undeclared crate or module `std`

This is because of no_std feature. But in this place (and a few others), it should be core::slice, not std::slice - it's a commonly confused thing it seems. Apparently this crate has never been test-run by cargo.

Another large source of errors is examples/ordered.rs:

error[E0277]: the trait bound `std::vec::Vec<i32>: CollectionRef` is not satisfied
  --> examples/ordered.rs:57:2
   |
57 |     ordered_stack_usage::<Vec<i32>>(); // a `Vec` is a stack so it works.
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `CollectionRef` is not implemented for `std::vec::Vec<i32>`
   |
   = help: the following other types implement trait `CollectionRef`:
             HashMap<K, V>
             HashSet<T>

error[E0277]: the trait bound `std::vec::Vec<i32>: PushBack` is not satisfied
  --> examples/ordered.rs:57:24
   |
57 |     ordered_stack_usage::<Vec<i32>>(); // a `Vec` is a stack so it works.
   |                           ^^^^^^^^ the trait `PushBack` is not implemented for `std::vec::Vec<i32>`
   |
note: required by a bound in `ordered_stack_usage`
  --> examples/ordered.rs:47:48
   |
45 |     fn ordered_stack_usage<S>()
   |        ------------------- required by a bound in this function
46 |     where
47 |         S: Default + Collection<Item = i32> + Back + PushBack,
   |                                                      ^^^^^^^^ required by this bound in `ordered_stack_usage`

error[E0277]: the trait bound `std::vec::Vec<i32>: Back` is not satisfied
  --> examples/ordered.rs:57:24
   |
57 |     ordered_stack_usage::<Vec<i32>>(); // a `Vec` is a stack so it works.
   |                           ^^^^^^^^ the trait `Get<usize>` is not implemented for `std::vec::Vec<i32>`, which is required by `std::vec::Vec<i32>: Back`
   |
   = help: the following other types implement trait `Get<T>`:
             HashMap<K, V>
             HashSet<T>
   = note: required for `std::vec::Vec<i32>` to implement `Back`
note: required by a bound in `ordered_stack_usage`
  --> examples/ordered.rs:47:41
   |
45 |     fn ordered_stack_usage<S>()
   |        ------------------- required by a bound in this function
46 |     where
47 |         S: Default + Collection<Item = i32> + Back + PushBack,
   |                                               ^^^^ required by this bound in `ordered_stack_usage`

error[E0277]: the trait bound `std::vec::Vec<i32>: Back` is not satisfied
  --> examples/ordered.rs:57:24
   |
57 |     ordered_stack_usage::<Vec<i32>>(); // a `Vec` is a stack so it works.
   |                           ^^^^^^^^ the trait `Len` is not implemented for `std::vec::Vec<i32>`, which is required by `std::vec::Vec<i32>: Back`
   |
   = help: the following other types implement trait `Len`:
             HashMap<K, V>
             HashSet<T>
   = note: required for `std::vec::Vec<i32>` to implement `Back`
note: required by a bound in `ordered_stack_usage`

etc.

For now, for virtiofsd needs, I disabled ijson, serde_json (lots of errors in there) and removed examples/ordered.rs, fixed s/std/core/ in a few places, and also patched out nightly feature. But apparently this can be fixed upstream for benefit of all :)