Adding custom_module sample.
This demonstrates how a user out-of-tree can use custom modules and
types with the IREE VM and includes some basic guided documentation.
There are still a lot of ergonomics improvements to be done (especially
with the C++ wrappers) but it's enough to implement the old custom
module sample but without needing any compiler changes at all (can
do all of this from a pip install of iree-compile).