Remove cosmos/iavl from lazyledger-core to stop dependency graph cycle

[evan-forbes]

Currently, we're trying to replace tendermint for lazyledger-core in the lazyerleger-app #4 and in the lazyledger fork of the cosmos-sdk #2. We run into the same error if we attempt to change the import by using either the go modules' replace directive, or by using a script to manually replace every import from "github.com/tendermint/tendermint" to "github.com/lazyledger/lazyledger-core".
error:

go: github.com/cosmos/cosmos-sdk/baseapp imports
        github.com/tendermint/tendermint/abci/types imports
        github.com/lazyledger/lazyledger-core/crypto/ed25519: github.com/lazyledger/[email protected]: parsing go.mod:
        module declares its path as: github.com/tendermint/tendermint
                but was required as: github.com/lazyledger/lazyledger-core

After digging through the dependency trees of the libraries, I found that lazyledger-core and the cosmos sdk are both importing different versions of the cosmos/iavl, and those two versions of the iavl each import their own version of tendermint. So, connecting the cosmos-sdk to lazyledger-core brings us to a special level of dependency hell, go.mod dependency hell, which stops go modules from reconciling the different module declariation of tendermint and its fork, lazyledger-core. better explained in this post.

I think we're going to have to simultaneously update our fork of the cosmos-sdk, lazyledger-core, and a new fork of cosmos/iavl to new non-existent versions in order to break the import cycle. as described here

[evan-forbes]

I also found another import cycle that stops go modules from updating the modules of lazyledger-core with go get -u ./..., that's caused by the recent switch from github.com/liamsi/merkletree to github.com/lazyledger/merkletree. Luckily, the import cycle is simpler, and can be fixed by updating the nmt module imported in lazyledger-core using go get -u github.com/lazyledger/nmt.

[tac0turtle]

Within lazyledger-core we can remove Iavl. It is used for testing light client queries but we will not be supporting these form of proofs (I believe). I can open a PR later today.

[liamsi]

It makes sense to remove IAVL from ll-core. AFAR, it is only used in tests only.

Linking related issues & discussions here for context:

• bump to a newer tendermint version #112 (comment)
• Circular dependency with IAVL?  tendermint/tendermint#5694
• Tendermint tmlibs and common dependencies cosmos/iavl#46
• update tendermint dependency to v0.30 cosmos/iavl#125 (comment)