Add ASN.1 encoders and decoders for SPAKE types

Add ASN.1 encoders and decoders for SPAKE types

Add a new internal header k5-spake.h.  Add ASN.1 encoder and decoder
functions and an internal free function for SPAKE types.  Add ASN.1
tests and asn1c test vectors the new types.

The additions to to make-vectors.c use C99 designated initializers in
order to initialize unions.  This is okay since make-vectors.c is only
compiled as part of "make test-vectors" and not as part of the regular
build.