WIP: Lifetimed MdbValue
Dropped DB handle trait, marked transaction traits as public

Later is a quick workaround to make it compilable with nightly
Bitflags for Environment and Database
WIP: db creation requires fixes
Make Emacs happier with macro indentation
Re-exporting value conversion traits
Added Item Cursor
Deletion of DB also moves value
Short descriptions for Database methods
Fixed Item Iterator

It initialised cnt incorrectly before going to first key and therefore can randomly skip correct data
Environment set/get flags uses bit flags too
Update to new cfg style
Temp: bool -> mdb value
Make `traits` public, added docs
Extracted tests into a separate mod
Environment improvements

- should be created using EnvBuilder

- uses `FilePermission` instead of `mdb_mode_t`

- makes distinction between flags available on creation and flags available after opening

- simplified code (no more state checks)
Explicit use of `ffi` items in core