Change usize to u64 for all variables that aren't indices into byte-based structures

[Cydhra]

Bit vectors can hold more bits than usize::MAX, and therefore all bit-indexing and length calculation should be performed with u64 instead.

This has some unfortunate side effects that need to be addressed

• ExactSizeIterator does not support iterators with more than usize::MAX elements. Code paths where this can be circumvented should be special-cased for <64 bit plattforms, and where it can't there should be documentation about potential panics.
• take() and skip() in iterators takes a usize, which means subtrees in BP that exceed 2^32 bits cannot be handled properly.
• When converting from u64 to usize for indexing byte-based structures, care must be taken that conversions happen in the correct order