MPT new descriptors & benchmark

[miha-stopar]

Description

New descriptors are used which have Vec<Node> structure and thus the conversion in witness_row.rs from [][]byte to Vec<Node> was removed. Note that keccak_data is now part of the Node struct - each node holds the byte streams that need to be hashed.

Additionally, an MPT benchmark was added (some structs needed to be made public).

Type of change

• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

Contents

• All tests updated.
• prepare_witness function removed from witness_row.rs.
• MPT test updated to not use prepare_witness.
• Benchmark added.

[Brechtpd]

Thanks a lot for doing the witness refactor!

I think there's something up with the benchmark, but otherwise looks great.