Add generic state verification functions to ClientState interface

[damiannolan]

Summary

Add two new generic state verification functions to the ClientState interface.

• VerifyMembership
• VerifyNonMembership

ref: cosmos/ibc#687

Proposal

Separate PRs can be made to split up the work:

• Add new VerifyMembership and VerifyNonMembership to ClientState interface
• 06-solomachine implementation
• 07-tendermint implementation
• 09-localhost implementation

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For Admin Use

• Not duplicate issue
• Appropriate labels applied
• Appropriate contributors tagged/assigned

[colin-axner]

For the solo machine implementation, to avoid being blocked by #1141, a switch case should be used on the provided path. If the path matches a supported type then the sign bytes can be constructed, otherwise an error should be returned. The implementation of #1141 will extend the generic verify functions for the solomachine to accept any path

[damiannolan]

I have a WIP branch for this from a few weeks ago so will self-assign!
PRs will be split up. I'll remove localhost from checklist

[colin-axner]

I can do the 07-tendermint changes