This commit introduces a new method of interacting with a TPM 2.0.
Instead of plumbing each TPM API into one or more Go functions, this
code defines structures for every TPM 2.0 command request and response.
These map 1:1 with the actual command parameters comprehended by the
TPM, so any invocation of any command is possible (once all the command
structures are written).

This commit introduces enough of the TPM 2.0 API surface to put together
some interesting end-to-end tests, mostly around sealing.

Another objective of the Direct API is to facilitate use of the TPM's
session-based command transport features (e.g., audit and encryption
sessions). See the test code for examples of how to easily use these
APIs to, e.g., set up an EK-salted session for session-encrypted unseal.

Change-Id: I1549dd596869d79ddd41ff3c5f9ffdadc9628ed4

* WIP: put stuff into subpackages to make the names nice

* use internal monolithic package to avoid cycles

* complete the tpm2 package

* fix test references to renamed types

* switch to command.Execute pattern

* fix some unkeyed field values detected by go vet

* stop panicking from Hash