Expect full denoms in Banker.IssueCoin and Banker.RemoveCoin

[thehowl]

From #875 (review):

That said, soon after this is merged, I think we'll need to change this API again. This current implementation creates an inconsistency within the Banker API. All other banker methods now require you to pass in the full realm path to the token you're referring to, but IssueCoin and RemoveCoin do not.

Thus, I think a few more changes are in order:

1. There should be a RealmDenom(pkgpath, denom string) function in std, which creates a realm denomination (ie. /gno.land/r/morgan:bitcoin). There can be a helper method Realm.Denom(denom string) (so you can do std.CurrentRealm().Denom("bitcoin")
2. Instead of modifying denom's value in the native function, we should check it matches what we expect. ie. strings.HasPrefix(denom, RealmDenom(std.CurrentRealm().PkgPath()), then check the last part of the denom to see that it matches the Gno regex. (This can all be done in gno, without needing to put it in native code)

Related with #1475 #1576