[core] Update crypto module documentation, suitable for rustdocs.
[core] Update crypto module documentation, suitable for rustdocs.

This change allows us to view the crypto code documentation via `cargo
doc`. Some of the section heading are moved around to keep similar
concepts together and for markdown formatting.

Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

Pull request: #469
Approved by: metadave

georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,nathenharvey/habitat,rsertelon/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,georgemarshall/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,nathenharvey/habitat,nathenharvey/habitat,georgemarshall/habitat