use buffer pool

[Fi3]

No description provided.

[rrybarczyk]

The Codec currently implements and exports the Buffer trait.

It should instead import use and reexport the Buffer trait from buffer-pool. Via a compile flag, we should instruct the codec to use the Buffer that is currently using either SlowAndCorrect or the one that is implemented in buffer-pool (that is faster but has some unsafe code). We should modify the sv2-ffi crate in order to use the codec with the SlowAndCorrect buffer, so at least at the beginning, we will use the safest implementation in Bitcoin Core.