`from_bytes` should take `len` as an argument?

[venkatarun95]

Currently if I have a byte with 7 bits, there is no clean way to convert that to a BitVec. So from_bytes should probably take len as an argument. Since this can be a breaking change, it might be better to create a different function instead

[marcianx]

The thing about passing a length argument is that we have to define the behavior when the length doesn't match the byte list (e.g. exceeds it the bounds). The behavior of BitVec::from_bytes(b).truncate(len) feels a bit more obvious from an API perspective. I could document this strategy in the from_bytes documentation if it's not as obvious. WDYT?