Add a profile_traits crate to reduce compile times.

[nnethercote]

A rebuild after touching components/profile/mem.rs now takes 48 seconds (and
only rebuilds profile and servo) which is much lower than it used to be.
In comparison, a rebuild after touching components/profile_traits/mem.rs takes
294 seconds and rebuilds many more crates.

This change also removes some unnecessary crate dependencies in net and
net_traits.

[hoppipolla-critic-bot]

Critic review: https://critic.hoppipolla.co.uk/r/4870

This is an external review system which you may optionally use for the code review of your pull request.

In order to help critic track your changes, please do not make in-place history rewrites (e.g. via git rebase -i or git commit --amend) when updating this pull request.

[Manishearth]

Reviewed files:

• components/compositing/Cargo.toml @ r1
• components/compositing/compositor.rs @ r1
• components/compositing/compositor_task.rs @ r1
• components/compositing/constellation.rs @ r1
• components/compositing/headless.rs @ r1
• components/compositing/lib.rs @ r1
• components/compositing/pipeline.rs @ r1
• components/gfx/Cargo.toml @ r1
• components/gfx/lib.rs @ r1
• components/gfx/paint_task.rs @ r1
• components/layout/Cargo.toml @ r1
• components/layout/layout_task.rs @ r1
• components/layout/lib.rs @ r1
• components/layout/parallel.rs @ r1
• components/layout_traits/Cargo.toml @ r1
• components/layout_traits/lib.rs @ r1
• components/net/Cargo.toml @ r1
• components/net/lib.rs @ r1
• components/net_traits/Cargo.toml @ r1
• components/net_traits/lib.rs @ r1
• components/profile/Cargo.toml @ r1
• components/profile/lib.rs @ r1
• components/profile/mem.rs @ r1
• components/profile/time.rs @ r1
• components/profile_traits/Cargo.toml @ r1
• components/profile_traits/lib.rs @ r1
• components/profile_traits/mem.rs @ r1
• components/profile_traits/time.rs @ r1
• components/script/Cargo.toml @ r1
• components/script/layout_interface.rs @ r1
• components/script/lib.rs @ r1
• components/servo/Cargo.lock @ r1
• components/servo/Cargo.toml @ r1
• components/servo/lib.rs @ r1
• ports/cef/Cargo.lock @ r1
• tests/unit/net/Cargo.toml @ r1
• tests/unit/net/lib.rs @ r1

---

Comments from the review on Reviewable.io

[Manishearth]

Reviewed files:

• components/profile_traits/time.rs @ r2

---

Comments from the review on Reviewable.io

[Manishearth]

@bors-servo: r+

[bors-servo]

📌 Commit 092507d has been approved by Manishearth

[bors-servo]

⌛ Testing commit 092507d with merge 77f653d...

[bors-servo]

☀️ Test successful - android, gonk, linux1, linux2, mac1, mac2