abis__compute_function_tree_root and abis__compute_function_tree should use vectors instead of raw buffers

[sirasistant]

Right now the circuits CPP expects the leaves to be a raw buffer:
https://github.com/AztecProtocol/aztec3-circuits/blob/fdd7a3577deb2bc6fd90bb68b16c654695331d08/cpp/src/aztec3/circuits/abis/c_bind.cpp#L252

https://github.com/AztecProtocol/aztec3-circuits/blob/fdd7a3577deb2bc6fd90bb68b16c654695331d08/cpp/src/aztec3/circuits/abis/c_bind.cpp#L265

This will be changed to admit a vector of leaves.
AztecProtocol/aztec3-circuits#203
When that is changed to a vector we should update our circuits.js implementation to also use vector:

https://github.com/AztecProtocol/aztec3-packages/blob/master/yarn-project/circuits.js/src/abis/abis.ts#L49
https://github.com/AztecProtocol/aztec3-packages/blob/master/yarn-project/circuits.js/src/kernel/kernel.ts#L24

[dbanks12]

Ticket that focuses on the C++ side of this: AztecProtocol/aztec3-circuits#203