Witness struct is in place of the Vec<Vec<u8>> we have before this commit.
from_vec() and to_vec() methods are provided to switch between this type and Vec<Vec<u8>>
Moreover, implementation of Default, Iterator and others allows to have similar behaviour but
using a single Vec prevent many allocations during deserialization which in turns results in
better performance, even 20% better perfomance on recent block.
last() and second_to_last() allows to access respective element without going through costly Vec
transformation
Riccardo Casatta