Allow 0-width regions, remove panic on invalid range
Allow 0-width regions, remove panic on invalid range

The validate function now returns a Result instead of panicking. This
means also find and insert will return a Result.

Zero-width regions can now be queried and inserted, but do not overlap
anything. This behaviour is the same as in Bioconductor's IRanges.

rust-bio/rust-bio,FrancisRussell/rust-bio,veldsla/rust-bio,pmarks/rust-bio