This commit introduces a new package `conformance` containing:
1. the test driver to exercise Lotus against interoperable
test vectors, and
2. the test runner, which integrates go test with the test vector
corpus hosted at https://github.com/filecoin-project/conformance-vectors.
The corpus is mounted via a git submodule.
Right now, only message-class test vectors are supported. In the
next week, this support will be extended to tipset-class, chain-class,
and block sequence-class vectors.
