refactor(x/mint): unexport keeper methods

[p0mvn]

Closes: #XXX

What is the purpose of the change

Currently, a lot of the keeper methods are exported when they are only used inside the keeper package or in tests.

For the purposes of information hiding, we should unexport as many methods as possible to avoid misuse from incorrect locations.

There are no changes other than unexporting some methods and none of the methods were removed.

Brief Changelog

• unexport setLastReductionEpochNum
• unexport getLastReductionEpochNum
• unexport createDeveloperVestingModuleAccount
• unexport mintCoins
• create a helper in apptesting for minting tokens in tests
• reshuffle order in mint keeper.go so that exported methods go first, then unexported

Testing and Verifying

This change is a trivial rework / code cleanup without any test coverage.

Documentation and Release Note

• Does this pull request introduce a new feature or user-facing behavior changes? yes
• Is a relevant changelog entry added to the Unreleased section in CHANGELOG.md? yes
• How is the feature or change documented? not applicable

[p0mvn]

Note to reviewers: some methods in this file were shuffled around so that exported methods go first, and, then, unexported follow

None of the methods were actually removed

[czarcas7ic]

LGTM. Is there any specific methodology you go through to determine which keeper methods should stay unexported? I think it may be helpful to document this somewhere so we can easily determine in the future (and possibly automate warnings!)

[p0mvn]

LGTM. Is there any specific methodology you go through to determine which keeper methods should stay unexported? I think it may be helpful to document this somewhere so we can easily determine in the future (and possibly automate warnings!)

As a general rule, we should be hiding as many methods as possible without a specific use case outside of the internal logic. More on the principle.

For example, with x/mint no other module should be able to MintTokens or CreateDeveloperVestingModuleAccount outside of the internal mint keeper logic so it makes sense to hide them.

We should be encapsulating logic into stable interfaces and exposing them to the clients. In the context of x/mint keeper, the core of these stable interfaces is AfterEpochEnd hook + some other boilerplate methods that all keepers implement. Everything else is internal logic and should be encapsulated by the keeper.