introduce interoperable vector-based conformance testing.

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.

.gitmodules

@@ -5,3 +5,7 @@
 [submodule "extern/serialization-vectors"]
 	path = extern/serialization-vectors
 	url = https://github.com/filecoin-project/serialization-vectors
+[submodule "extern/conformance-vectors"]
+	path = extern/conformance-vectors
+	url = https://github.com/filecoin-project/conformance-vectors.git
+	branch = initial