[zx] Magenta -> Zircon

The Great Renaming is here!

Change-Id: I3229bdeb2a3d0e40fb4db6fec8ca7d971fbffb94

[utest] run magenta tests in groups

Assign a group to magenta tests, and run tests in order of
increasing complexity, and abort the test if a group failed.
This is because if there is a bug in a core magenta facility,
it could cause all higher level tests to fail.

The current groups are:

* "core": tests on kernel facilities
* "ddk": ddk tests
* "sys": general magenta system tests (default)
* "fs": filesystem tests

Set the group in the tests' rules.mk by setting the
MODULE_USERTEST_GROUP variable.

runtests accepts a list of groups to run. If none is specified,
run all tests.

Change-Id: Ia363d25f5c1c828a3d1a9abf586170bb90c679f7

[build] flatten the build

Previously we treated kernel/, system/, and third_party/ as
overlays on a shared namespace. This required the concept
of "canonical" module names, and a lot of complexity to ensure
that things didn't collide and the build worked.

This change gets rid of that, no longer passes -I to make,
so that include directives from our *.mk files do not magically
wildcard across various paths, etc.

The most user-visible change is that everywhere where a module
name is specified (MODULE_DEPS, MODULE_LIBS, etc), full module
names like kernel/lib/io or system/ulib/mxio must be used instead
of previously-allowed "short" names like lib/io and ulib/mxio.

The build output still has a similar shape, but the first segment
of the module path (kernel/, system/, or third_party/) is no
longer elided under $(BUILDDIR)

Change-Id: I525aba1da1c86eb7a86007bddc669f7eeebfedd5

[object] add mx_object_set_cookie(), mx_object_get_cookie()

Initially cookies are supported on Events, VMOs, and Resources.

Change-Id: Iae0c9aaf6f05177be6872e5aa00774932926b4d1