Since the decryption was done in place, if an incorrect key was given,
the encrypted data would be corrupted.
Fixesgolang/go#8439.
R=bradfitz
CC=golang-codereviews
https://golang.org/cl/115550043
Manual edits to README.
Moved from main Go repository, deleted Makefiles, ran gofix -r go1rename.
Tested with: go test code.google.com/p/go.crypto/...
R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5564059