perf: store HashBuilder.value as a Vec<u8> and track kind separately

[DaniPopes]

The current API of HashBuilder::leaf creates a new allocation for each new value. This is wasteful, and should instead re-use an internal buffer like we're doing for RLP.

Practically, this means replacing value with a Vec<u8> and a separate flag to track whether it is a hash or a leaf value. The add_* APIs should remain the same, and internally they will clear the buffer, copy the new value in, and set the flag.