It turns out that dclone is slower than just calling uint128 repeatedly

I'd revert the previous commit but I want to keep the method rename.
Replace some bit shifting with a string index operation

Doing a lot of bit shifting on Math::Int128 objects is kind of slow, so this
is a nasty hack to turn the number into a bitstring and then find the first 1
bit by using index().

This shaves about 10% off building a 50,000 network tree
Small optimization for File->_encode_record

We were calculating some values on every call that we only needed to calculate
once, and it turns out that Math::Round::round is a little slow.