share: rethink Share type

[Wondertan]

          i think it will be nicer for user to use those as methods attached to `Share` type. Compare:

namespace := myshare.Namespace()

vs

namespace := share.Namespace(myshare)

Originally posted by @walldiss in #2388 (comment)

[Wondertan]

Definitely, and I tried that a long ago, but then there is type conversion that we have to do everywhere from []Share to [][]byte. Unfortunately, Go does not do that automatically, and there are too many conversions we have to make. When it's an alias, it's converted automatically. I am still open and want them to be methods and we should open an issue. But the only way to fix conversions is to make NMT and RSMT2D use the Share type, which is controversial.

[Wondertan]

Also, the app team has its own definition of share, which we should eventually unify with and analyze their choices