Feedback on new crypto primitives and API

[Foxboron]

Yo,

I'm poking a bunch of people that is currently using this library to help provide feedback before I remove the old code. I assume people are relying on this code in different manners and It's hard for me to keep track.

• pkcs7 and authenticode is rewritten with x/crypto/cryptobyte which is a lot easier to understand and deal with then the old struct based ASN.1 code.
• authenticode appends signatures and rewrites the binary with several SectionReader and io.MultiReader. I suspect this is the less clever part of the new code instead of reading the file as a byte slice and doing surgery on the file that way.
• efivarfs is a new abstraction to deal with efivarfs.
• efivarfs/testfs now has several abstractions to support an in-memory efivarfs for testing purposes.
• tests is rewritten with vmtest for EDKII/tianocore integration testing.

New code example to sign a binary in the README.md.

Feel free to come with opinions and requests for the rewritten code :)

@zaolin @smoser @werwurm @malt3 @flanfly @robertfairhead @smira @edgrz