Make some default mint coins restrictions

[ValarDragon]

Summary

We need a mintcoins restriction function creator, for easily creating restrictions to only mint coins with the provided prefix. E.g.

// NewPrefixMintCoinsRestriction creates a mint coins restriction fn, that only allows banks to mint coins that have a denom beginning with "...."
func NewPrefixMintCoinsRestriction(prefix string) MintCoinsRestrictionFn {
...
}

Problem Definition

We're going to use this same code for gamm and token factory.

Blocked on #48

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[mattverse]

Should I create a PR against #50 branch for this issue?What Im thinking as of now is adding a default mintcoin restriction function creator together with #50 and get it merged and reviewed together.

[mattverse]

Also do we have an idea on what default minting restriction we want? I'm currently thinking of making a variadic function taking in strings that you would want to restrict minting on

[ValarDragon]

oh interesting idea, I think that makes sense. Basically emits a restriction fn, that blocks emitting anything that starts with any of the variable inputs.

[ValarDragon]

Closed in #57