Authenticity of the scheme manager files in the irma_configuration
folder is now checked as follows.
Each scheme manager must henceforth include three new files in its
root folder: index, index.sig, and pk.pem. index lists all files
under this manager as well as their SHA256-hash. index.sig is a
ECDSA signature over index, signed with the private key corresponding
to the public key pk.pem. Thus, a file under the scheme manager
is authenticated if its hash matches the one in the index file,
and if the signature over the index file is valid.

Authenticity of all xml files, including the Idemix public keys,
is verified in this way automatically when these files are loaded.
For issuer and credential logos, use Configuration.ReadAuthenticatedFile.

This feature can no longer work because the scheme managers in the
irma_configuration of the old Android app are not signed, so they
would be rejected anyway.

The struct name now matches exactly (up to idiomatic spelling differences)
with the name of the folder that it parses and updates.