v0.5.0

This release took quite some time but I'm very happy of all the changes.
The api is now macro free. It took a few tries to get to this api for workloads but it works now. There is just a rustc weird behavior that makes it impossible to use values for workload systems. It's an acceptable price to get rid of the macro but it'd be great if it could be fixed.

Most notable changes:

• Remove system! macro
  This macro was used to hide what information workloads need. For example using system!(check) expended to (|world: &World| world.try_run(check), check)).
  The scheduler would use the function on the right to define what the function on the left borrows. If the two functions didn't match you could run into runtime errors.

  The new api (.with_system(&check)) does not have this problem.

• Remove packs
  Packs are removed temporarily, this implementation had too many limitations. The next implementation should not have the same problems but requires a fairly big amount of work so this version ships without packs and the next version should add them without modifying any existing api.

  remove was the most impacted, to remove two or more components you had to write Remove::<(usize, u32)>::remove((&mut usizes, &mut u32s), entity);.
  With 0.5 it's just (&mut usizes, &mut u32s).remove(entity).

• Bulk add entity
  This new method is faster than adding entities one by one.

• Remove Shiperator
  0.4 used a custom Iterator trait to modify some methods' behavior like filter.
  With 0.5, iterating an exclusive storage yields a Mut<T> by default which removes the need for a custom trait.

• Accurate modification tracking by default
  In 0.4 when you tracked modification on a storage it would tag all components accessed exclusively regardless if they were modified or not.
  This is not true anymore, this code would tag exactly what you would expect:

  fn increment_even(mut u32s: ViewMut<u32>) {
      for mut i in (&mut u32s).iter() {
          if *i % 2 == 0 {
              *i += 1;
          }
      }
  }

• No more try_*
  In 0.4 all fallible functions had two versions, one that panicked and one prefixed with try_ that returned a Result.
  Now all functions that can fail because of storage access return a Result while almost all others panic.

• Workload building is more flexible
  In 0.4 you could only create workloads directly with World and it was borrowed for the entire building time.

  world
      .add_workload("Add & Check")
      .with_system(system!(add))
      .with_system(system!(check))
      .build();

  In 0.5 you only need to borrow World to actually add the workload to it.

  Workload::builder("Add & Check")
      .with_system(&add)
      .with_system(&check)
      .add_to_world(&world)
      .unwrap();

  You can also "prepare" systems in advance, merge WorkloadBuilders, extend a WorkloadBuilder with a system iterator or even nest multiple workloads.

• Workload debug
  When you create a workload you can print information about how it will be scheduled and why two systems can't run at the same time.

• Custom view
  A lot of internal traits, types and functions are exposed in 0.5. One of them is Borrow, it allows user defined views. Custom views are very important to build clean apis on top of Shipyard.
  For example this custom view (https://github.com/dakom/shipyard-scenegraph/blob/master/crate/src/views.rs#L10-L28) borrows 11 storages. Users won't have to borrow 11 views though, just a single one.
  Creating a custom view is currently fairly verbose, I want to add a proc macro that would automatically derive the necessary traits.

• Custom storage
  Shipyard comes with two default storages: SparseSet and Unique. 0.5 allows you to define your own storages. You can then build a view for them and access them in systems for example.
  The advantage compared to simply storing your type in Unique is to be able to define what your storage should do when an entity is deleted.