Changes for version 0.16
- OnDisk: Close the file after creating the mmap(), the kernal will take a refcount on the file internally. This avoids running out of file descriptors when mapping many files. Thanks to Luciano Rocha for noticing this. Doc tweaks and cleanup changes given that 0.15 was a non backwards compatible release.
construct a "two level" minimal perfect hash
construct or tie a "two level" minimal perfect hash based on disk