A number of small bug fixes, some motivated by compiler warnings.

  * key-data.c:key_nextsubkey -- explicitly return nonzero if we found
    something.

  * key-io.c:key_new -- cast the constant type pointer during the
    unpleasant hack.

  * mp-mem.c:mp_build -- store an arena in the built integer; otherwise
    pgen_primep (for example) gets confused later on.

  * mp-modsqrt.c:mp_modsqrt -- fix the maths in a comment; the code was
    fine.

  * oaep.c:oaep_decode -- don't try to do too much in one expression.

  * pgen-simul.c:pgen_simultest -- always return a sensible result code.

  * cc.h:sig -- hash classes are constant.

  * cc-{kem,sig}.c:get{kem,sig} -- initialize the kp structure member,
    just in case.

  * rijndael*.c, square.c -- fix const-correctness errors.