Cleanup: introduce word256 and remove dependency on tendermint/go-common

[benjaminbollen]

While working on release-0.16, to have the eris-db definitions for Permissions to be a clean import into eris tooling, I removed the lingering dependency on tendermint/go-common; this saw me forced to pull this new type through the EVM and state as well; and finally there was then no need to preserve tendermint/go-common for sanity checks and random numbers for tests.

• introduces eris-db/word256 for bytes32 and padding and conversion functions

• introduces eris-db/common/sanity for wrapping PanicSanity and PanicCrisis

• introduces eris-db/common/random for wrapping around rand and crypto/rand

• changes imports across eris-db/manager and eris-db/permissions

[silasdavis]

This is great news.

It looks like this is all extracted from go-common, which has no tests. So it doesn't make matters any worse but not adding them so I'm approving.

So I know you know this, but I'm going to say it anyway: this stuff is so unit testable, wtf are the tests?

[benjaminbollen]

Yes, I agree, particularly by separating it into a word256 instead of a bigger common bag; it makes it very clear what the unit testing for it should be