Integrate with frontend

[Kevingislason]

This is the MVP version of Manta Signer. Manta Signer is a desktop application composed of a simple UI and a daemon running in the background. When Manta's web app needs to perform an operation requiring access to the root seed, it sends a request to Manta Signer's daemon.

Manta Signer has two purposes:

1. Increase ZKP generation speed--native Rust is much faster than web assembly (for now at least)
2. Protect Manta account secrets by keeping them out of the browser

When connected to a locally-running instance of Manta's web app, users are prompted to create an account on first login. Afterwards, the web app can use Manta Signer's daemon to generate addresses, mint private assets, reclaim private assets, and transfer private assets. Operations that spend private assets (reclaims and private transfers) prompt a Manta Signer popup requesting password entry.

-Reclaims and private transfers are temporarily broken pending Merkle tree changes.
-This only works on MacOS for now
-Need to revert back to the manta branch of manta-api once the ark-0.3 change is merged.

[stechu]

LGTM in general pending handling errors and a few nits.
Testing seems to be a big issue here. I think we perhaps want to log some known issues so that we can at least remember to fix them in the future.

[stechu]

PS: Please add a PR description as well.

[stechu]

One more thing, please remove the .a file from the PR. It should be generated from the build rather than commited.

[Kevingislason]

@stechu Can you change the name of this repo to `manta-signer?

[stechu]

@stechu Can you change the name of this repo to `manta-signer?

Done, also manta-api changes have been merged.