Replace deprecated rust-crypto dep with RustCrypto org packages

[adizere]

Closes: #352
Closes: #720

Description

Our security audits are starting to fail:
https://github.com/informalsystems/ibc-rs/runs/2006092577?check_suite_focus=true

After a brief search, I choose to use crates from https://github.com/RustCrypto. Their crates seem to be maintained (repos had activity within the last few hours/days).

• Note that we're still depending on rust-crypto transitively via bitcoin-wallet v1.1.0. hence the security audit still fails. h/t @romac for suggesting tiny-bip39 to replace bitcoin-wallet.
• Also removed dependency ics23 since we didn't use it and triggered audit failure (see RUSTSEC-2020-0146: arr! macro erases lifetimes #720). Maintainers of ics23 are aware of the problem (ics23/#37).

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For contributor use:

• Updated the Unreleased section of CHANGELOG.md with the issue.
• If applicable: Unit tests written, added test to CI.
• Linked to Github issue with discussion and accepted design OR link to spec that describes this work.
• Updated relevant documentation (docs/) and code comments.
• Re-reviewed Files changed in the Github PR explorer.

[romac]

As we only use bitcoin-wallet for BIP39 mnemonics, I suggest we switch over to https://lib.rs/crates/tiny-bip39 which provides just the functionality we need. Of course we should first check if that crate passes the cargo-audit check.