sigscheme: Interface for signature schemes

sigscheme: Interface for signature schemes

There are no implementations yet.  This is split out like this for
review of the api.

We use struct buffer_if to feed key data into the scheme, in both
cases.  This will be convenient for implementations, The loadpub
caller will have the data from base91s, probably.  The loadpriv caller
will have it from a file, but we read the file in the common code.

The API we are introducing now expects each private key buffer to be
fed to the sig schemes one by one until they it finds one that likes
it.  This is in fact not necessary; it was needed in an earlier design
which does not otherwise survive in the published git history.  This
rather suboptimal API will be changed later.

Signed-off-by: Ian Jackson <ijackson@chiark.greenend.org.uk>