Add a toolchain target spec for AArch64

Add an outline document for a port of the Redox kernel to the Arm AAr…

Also fix a typo with the device_crate URL.

kill: support signal number 0

Tests process existence, but does not send a signal. Matches POSIX
behavior.

Fix delivery of signals when a signal uses the default handler, make context status update on every switch

Fix memory leaks in exec
Remove warnings

Use slab allocator for kernel heap

Operate on word size as possible

Fix #79 by limiting mappable sections to the 2GB mark