bitset: add from with length func

[paralin]

Allowing to reconstruct a bitset from the outputs of Len() and Bytes().

[lemire]

I do not understand the need for this function?

[paralin]

I want to store the BitSet as []uint64 (output of Bytes) and Length so that I can reconstruct an identical BitSet later

Not seeing a way to do that with existing APIs, the From will construct one with length == uint(len(buf))*64 which is different from the original Length.

[lemire]

Why won't From(set) do?

[paralin]

From will construct one with length == uint(len(buf))*64 which is different from the original Length.

[lemire]

Running tests.