Read-only support

[bovee]

We need some way to support opening a bit-vector in read-only fashion. We had a readonly parameter on MmapBitVec::open at one point, but this doesn't work any more because upstream memmap split their base struct into mutable and non-mutable variants without a common trait to unify them (and passing a file created with OpenOptions::new()...write(false)... to a MmapMut initializer doesn't work either).

There are a few possible ways to do this:

• implement our own trait on MmapMut and Mmap and store a boxed version of either in the MmapBitVec struct (this may give a perf hit?)
• create a separate MmapBitVecReadOnly struct that implements BitVector too and let downstream choose
• fork memmap :/