hrp len const fn

[tzemanovic]

makes the Hrp::len a const fn

[clarkmoody]

ACK 72ad45a

[tcharding]

What is the motivation? Just because we can and its better? If so perhaps we do FieldVec::len at the same time. I checked all the other len functions and they are all ExactSizedIterator trait methods so cannot be const.

[apoelstra]

Every pure function should be const, unless there are compiler limitations (such as them being trait methods) that prevent it, or we expect to run into compiler limitations with future refactorings.

[apoelstra]

ACK 72ad45a; successfully ran local tests