WIP: Add rayon par_iter; add/remove trace!() calls

Note that this commit ends up with a page fault, which I've traced to
line 127 of service_tree.rs. I'm adding this commit so @jackpot51 can give
me a hand.
2 jobs for dependencies in 2 minutes and 35 seconds
Status Job ID Name Coverage
  Build
failed #8209
allowed to fail
build:linux:cargo

00:01:36

passed #8210
build:redox:cargo

00:02:35

 
Name Stage Failure
failed
build:linux:cargo Build

error: aborting due to 6 previous errors

For more information about this error, try `rustc --explain E0308`.
error: Could not compile `init`.

Caused by:
process didn't exit successfully: `rustc --edition=2018 --crate-name init src/main.rs --color never --crate-type bin --emit=dep-info,link -C debuginfo=2 -C metadata=ee3772e98b67224c -C extra-filename=-ee3772e98b67224c --out-dir /builds/redox-os/init/target/debug/deps -C incremental=/builds/redox-os/init/target/debug/incremental -L dependency=/builds/redox-os/init/target/debug/deps --extern chashmap=/builds/redox-os/init/target/debug/deps/libchashmap-0bcf87b7bd504d7f.rlib --extern failure=/builds/redox-os/init/target/debug/deps/libfailure-331543b8f6bdf7b2.rlib --extern generational_arena=/builds/redox-os/init/target/debug/deps/libgenerational_arena-b3e75122ac189bce.rlib --extern log=/builds/redox-os/init/target/debug/deps/liblog-ea809242742fb90a.rlib --extern rayon=/builds/redox-os/init/target/debug/deps/librayon-69b55ed1edfbe25c.rlib --extern syscall=/builds/redox-os/init/target/debug/deps/libsyscall-32ac460d1641bc61.rlib --extern serde=/builds/redox-os/init/target/debug/deps/libserde-406365acb0e982bc.rlib --extern serde_derive=/builds/redox-os/init/target/debug/deps/libserde_derive-95eb8f153eb41f9d.so --extern simple_logger=/builds/redox-os/init/target/debug/deps/libsimple_logger-892fcd632f9953b5.rlib --extern toml=/builds/redox-os/init/target/debug/deps/libtoml-a2dbfaccde32f07c.rlib -L native=/builds/redox-os/init/target/debug/build/backtrace-sys-afabf112898a7cd7/out` (exit code: 1)
ERROR: Job failed: exit code 1