Remove composite literal uses of unkeyed fields

[husio]

This is a fresh take on go vet discussion. Previous story is #229 (comment)

We require go vet to pass as part of our test suite. We run it with -composites=false. This is a compromise because we used to have a huge amount of unkeyed fields, mostly in test files.

I have checked it again and today there is very little left.

$ go vet github.com/iov-one/weave/... 2>&1 | wc -l
42
$ go vet github.com/iov-one/weave/... 2>&1 | grep -v examples | wc -l
11

Acceptance criteria

• replace all unkeyed fields with recommended assignment
• remove -composites=false flag from go vet in Makefile