Optimisation: don't copy memory in proof deserialization

[gballet]

The issue is explained in this comment:

https://github.com/gballet/go-verkle/pull/175/files#r784676273

This optimization assumes that the buffer won't be reused at a later stage. Because the go-ethereum part of the code is still very much work in progress, we decided to go for the slower, safer route and copy the data. To be revisited when the API becomes more stable.