Move libsnark in-repo as a git subtree
This PR pulls in the libsnark subtree at the exact commit that we currently fetch via the depends system. To verify:
```
$ ./contrib/devtools/git-subtree-check.sh src/snark
src/snark in HEAD was last updated to upstream commit 9ada3f84ab484c57b2247c2f41091fd6a0916573 (tree c10a38c759)
src/snark in HEAD currently refers to tree 34e916d3f6
:100644 100644 427f4f4ce913e54da68b M Makefile
:040000 040000 42f29e42d1dd73536163 M src
FAIL: subtree directory tree doesn't match subtree commit tree
```
This shows that there are changes relative to what we currently use, due to the later commits in the PR. If we exclude them, we see that the code is identical:
```
$ git checkout 26a8f68ea8
$ ./contrib/devtools/git-subtree-check.sh src/snark
src/snark in HEAD was last updated to upstream commit 9ada3f84ab484c57b2247c2f41091fd6a0916573 (tree c10a38c759)
src/snark in HEAD currently refers to tree c10a38c759
GOOD
```
Closes#820.
[Test] New merge test suite driver script
Running the script with no arguments will run all test stages in succession.
Passing one or more stages as arguments will run just those stages.
Closes#429.
Not moved, because upstream makes improvements to this script, and the need to
set environment variables makes it simpler to just use the given script.
Migrate MiniNode to Zcash
This enables various RPC tests that use it (most of them in the extended test suite) to properly test Zcash code.
The PR also fixes bugs in the BIP65 and BIP66 tests that were both masking and masked by the un-migrated MiniNode.
The Python module `pyblake2` is now a requirement for the RPC tests.
Part of #2530.
