cleaner kernel-user exchange area manipulation

[pthierry-ledger]

Based on crate-local trait implementation for uapi types only.
Allows to properly define which (UAPI) type can be emited to or received from the kernel.
Make the exchange area interaction fully opaque

• cleaning exchange and define trait for systypes
• make exchange module private to the uapi crate
• validate C usage of the ffi_c copy_from_user/copy_to_user interface with autotest
• validate behavior in full rust mode with complete tests
• update uapi documentation accordingly

Note: to be rebased as soon as PR #101 is merged
Note2: the ffi_c is invalid by now, as the src_ptr, length is not properly manipulated. We may copy to a string and implement the trait for the String type in order to allow C-based *u8 data copy