Rename package to zvm-validator from libvalidator0
Update changelog to document package renaming

The validator isn't just a library; it also includes a couple of command
line tools for performing validation. But calling it just `validator` is
a bit ambiguous.

If this change is acceptable, this could set precedence for package
naming conventions for the other ZeroVM projects (`zvm-toolchain`,
`zvm-runtime`, etc.).
Merge pull request #12 from larsbutler/rename-package

Rename package to zvm-validator from libvalidator0