bench build fails

Building the benchmark part does not work, even on nightly.
```
cargo 1.42.0-nightly (ad3dbe10e 2020-01-13)
```

Here's the output I am getting:

```rust
Compiling legion v0.2.1 (C:\Users\[redacted]\Documents\GitHub\legion)
error[E0277]: the trait bound `itertools::groupbylazy::Group<'_, u8, std::slice::Iter<'_, Variants>, [closure@benches\parallel_query.rs:54:45: 54:59]>: std::iter::ExactSizeIterator` is not satisfied
--> benches\parallel_query.rs:58:17
|
58 | / group.map(|x| {
59 | | if let Variants::AB(a, b) = x {
60 | | (*a, *b)
61 | | } else {
62 | | panic!();
63 | | }
64 | | }),
| |__________________^ the trait `std::iter::ExactSizeIterator` is not implemented for `itertools::groupbylazy::Group<'_, u8, std::slice::Iter<'_, Variants>, [closure@benches\parallel_query.rs:54:45: 54:59]>`
|
= note: required because of the requirements on the impl of `std::iter::ExactSizeIterator` for `std::iter::Map, [closure@benches\parallel_query.rs:54:45: 54:59]>, [closure@benches\parallel_query.rs:58:27: 64:18]>`
= note: required because of the requirements on the impl of `legion::world::ComponentSource` for `legion::world::ComponentTupleSet<(A, B), std::iter::Map, [closure@benches\parallel_query.rs:54:45: 54:59]>, [closure@benches\parallel_query.rs:58:27: 64:18]>>`
= note: required because of the requirements on the impl of `legion::world::IntoComponentSource` for `std::iter::Map, [closure@benches\parallel_query.rs:54:45: 54:59]>, [closure@benches\parallel_query.rs:58:27: 64:18]>`

error[E0277]: the trait bound `itertools::groupbylazy::Group<'_, u8, std::slice::Iter<'_, Variants>, [closure@benches\parallel_query.rs:54:45: 54:59]>: std::iter::ExactSizeIterator` is not satisfied
--> benches\parallel_query.rs:68:17
|
68 | / group.map(|x| {
69 | | if let Variants::AC(a, c) = x {
70 | | (*a, *c)
71 | | } else {
72 | | panic!();
73 | | }
74 | | }),
| |__________________^ the trait `std::iter::ExactSizeIterator` is not implemented for `itertools::groupbylazy::Group<'_, u8, std::slice::Iter<'_, Variants>, [closure@benches\parallel_query.rs:54:45: 54:59]>`
|
= note: required because of the requirements on the impl of `std::iter::ExactSizeIterator` for `std::iter::Map, [closure@benches\parallel_query.rs:54:45: 54:59]>, [closure@benches\parallel_query.rs:68:27: 74:18]>`
= note: required because of the requirements on the impl of `legion::world::ComponentSource` for `legion::world::ComponentTupleSet<(A, C), std::iter::Map, [closure@benches\parallel_query.rs:54:45: 54:59]>, [closure@benches\parallel_query.rs:68:27: 74:18]>>`
= note: required because of the requirements on the impl of `legion::world::IntoComponentSource` for `std::iter::Map, [closure@benches\parallel_query.rs:54:45: 54:59]>, [closure@benches\parallel_query.rs:68:27: 74:18]>`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0277`.
error: could not compile `legion`.
warning: build failed, waiting for other jobs to finish...
error[E0308]: mismatched types
--> benches\benchmarks.rs:57:29
|
57 | Box::new(|e, w| w.add_component(e, A(0.0))),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found enum `std::result::Result`
|
= note: expected unit type `()`
found enum `std::result::Result<(), &'static str>`

error[E0308]: mismatched types
--> benches\benchmarks.rs:58:29
|
58 | Box::new(|e, w| w.add_component(e, B(0.0))),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found enum `std::result::Result`
|
= note: expected unit type `()`
found enum `std::result::Result<(), &'static str>`

error[E0308]: mismatched types
--> benches\benchmarks.rs:59:29
|
59 | Box::new(|e, w| w.add_component(e, C(0.0))),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found enum `std::result::Result`
|
= note: expected unit type `()`
found enum `std::result::Result<(), &'static str>`

error[E0308]: mismatched types
--> benches\benchmarks.rs:61:29
|
61 | Box::new(|e, w| w.add_component(e, D(0.0))),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found enum `std::result::Result`
|
= note: expected unit type `()`
found enum `std::result::Result<(), &'static str>`

error[E0308]: mismatched types
--> benches\benchmarks.rs:63:29
|
63 | Box::new(|e, w| w.add_component(e, E(0.0))),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found enum `std::result::Result`
|
= note: expected unit type `()`
found enum `std::result::Result<(), &'static str>`

error[E0308]: mismatched types
--> benches\benchmarks.rs:65:29
|
65 | Box::new(|e, w| w.add_component(e, F(0.0))),
| ^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `()`, found enum `std::result::Result`
|
= note: expected unit type `()`
found enum `std::result::Result<(), &'static str>`

error[E0061]: this function takes 2 parameters but 1 parameter was supplied
--> benches\benchmarks.rs:165:19
|
165 | world.insert(
| ^^^^^^ expected 2 parameters

error[E0618]: expected function, found `(Tag,)`
--> benches\benchmarks.rs:166:17
|
166 | (Tag(i as f32),)
| _________________-^^^^^^^^^^^^^^^
167 | | (0..10000).map(|_| (Position(0.), Rotation(0.))),
| |__________________________- call expression requires function
```