Reevaluate dependency tree for tezedge/p256 functionality

[wyc]

As per #98 (comment)

If we selected dependencies carefully here, it may be possible to remove libsodium and slim the cryptographic profile of our dependency tree.

[sbihel]

The dependency on libsodium and tezedge-client was removed from non-test code. For now we rely on libssi and our implementation for things like key decoding. We will reevaluate the situation in the future as tezedge-client comes to maturity (e.g. we want to eventually use it for DID Manager origination and updates). This will improve the separation of concerns, but for now it works and makes the WASM compilation easier. Closing for now.