Move assembly into its own sub-module

In order to create a "convenience tar" it is better to create a seperate sub-module for assembly, as if it's in the parent pom, it will be built first, and the binaries will not necessarily be available at the time (only if it was built prior). Even if it is available, I can't refer to the artifact from assembly descriptor.

Bonus: add automatic checksum generation for every artifact created by maven. sha512 should be used. md5 is deprecated and sha1 is only 20 bytes.

For now, I will not backport this to 3.4