20-transfer: use special prefix for denominations

[cwgoes]

Clarify that the IBC module needs a subset of the denomination space that only it can create tokens in, and use one.

[colin-axner]

Does Clarify imply enforcing this via code or documenting it? We currently use ibc/ for cross chain token denom.

Enforcing via code sounds like injecting custom logic into bank or using capabilities. Are there any other potential solutions here?

[cwgoes]

Right. I guess it would require something like x/bank exporting special versions of MintCoins which could only mint coins for a particular prefixed subspace of coin denominations and giving the prefixed minter for ibc/ to the 20-transfer module, then prohibiting the / character in regular denominations. That's probably too large a refactor for now but we should at least document that 20-transfer requires this for safety even though it isn't enforced explicitly (just coincidentally not possible on the Hub).