[ENG-256] Crypto Crate Refinements (#410)

* add experimental preview media support

* clippy+formatting

* add support for mk/pvm nonces

* fix cursor positioning+size deserialization bug

* clippy+formatting

* cleanup code and add `decrypt_preview_media()`

* update comment

* remove `Mode` from `Keyslot`

* use little-endian `usize` for media length

* remove memory objects

* remove mode enum

* add `.zeroize()` to `Protected<>`

* remove `Mode` ser/de

* refactor, remove `Mode`, add helper functions

* formatting

* remove unused error

* update comments/docs

* comments, fix large files, insane performance

* doc updates and fix decrypt zeroize

* revert to 1048576 byte `BLOCK_SIZE`

* `Keyslot` and `PreviewMedia` constructors now handle encryption

* add metadata item with associated functions

* update comments, clean up useless items

* add metadata support within the header

* remove stray 128 in length calcs

* include metadata in header ser/de

* fmt+clippy

* rework keyslot decryption

* formatting

* api changes, code cleanup

* docs for `stream.rs`

* massive documentation re-write

* mark expensive doc test as ignore

* minor api tweaks

* add examples

* formatting+linting

* finalise documentation and add more examples

* formatting
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