update version to 0.5.0 (#764)

1.0.0 was never an intentional release and the nim-eth repo is not yet
stable in terms of API, thus we start at 0.5.0 and go from there for the
first tagged release.

Most of the eth code in this repo now is aligned with various specs
meaning that most API can be considered "mostly" stable, but there are
still aspects being worked on as well as a potential future
reorganisation of the code turning nim-eth into a more "pure"
spec-driven core ethereum infrastructure repo, removing in the process
application-level stuff that happens to be common between nimbus-eth1
and eth2 and ended up in here for convenience.