This library is not long for this world. Please do not add new things to it. Bug: https://fuchsia.atlassian.net/browse/ZX-2497

The plan is to split this library up into several parts:

1. Most of the headers in include/ddk/ should be moved to ulib/driver (which itself will be relocated to dev/lib/driver).
2. io-buffer, mmio-buffer, and phys-iter should be moved into dev/lib/io-buffer as a static library
3. The headers in include/hw should be moved to per-device-type libraries