Tantivy used to assume that all files could be somehow memory mapped. After this change, Directory return a `FileSlice` that can be reduced and eventually read into an `OwnedBytes` object. Long and blocking io operation are still required by they do not span over the entire file.
* add checksum check in ManagedDirectory
fix#400
* flush after writing checksum
* don't checksum atomic file access and clone managed_paths
* implement a footer storing metadata about a file
this is more of a poc, it require some refactoring into multiple files
`terminate(self)` is implemented, but not used anywhere yet
* address comments and simplify things with new contract
use BitOrder for integer to raw byte conversion
consider atomic write imply atomic read, which might not actually be true
use some indirection to have a boxable terminating writer
* implement TerminatingWrite and make terminate() be called where it should
add dependancy to drop_bomb to help find where terminate() should be called
implement TerminatingWrite for wrapper writers
make tests pass
/!\ some tests seems to pass where they shouldn't
* remove usage of drop_bomb
* fmt
* add test for checksum
* address some review comments
* update changelog
* fmt
