Zcash Browser Wallet Library

[tokidoki]

@Wollum-ChainSafe

The memory backend would become an officially supported wallet backend (useful for web or embedded applications where sqlite isn’t supported) and WebZjs would depend only on the official librustzcash fork which reduces the maintenance burden.

I think this should be done asap to limit any possible fragmentation to librustzcash, an important part of the Zcash stack.