PROV: Add the beginning of a DER writing library

PROV: Add the beginning of a DER writing library

This library is meant to be small and quick.  It's based on WPACKET,
which was extended to support DER writing.  The way it's used is a
bit unusual, as it's used to write the structures backward into a
given buffer.  A typical quick call looks like this:

    /*
     * Fill in this structure:
     *
     * something ::= SEQUENCE {
     *     id OBJECT IDENTIFIER,
     *     x [0] INTEGER OPTIONAL,
     *     y [1] BOOLEAN OPTIONAL,
     *     n INTEGER
     * }
     */
    unsigned char buf[nnnn], *p = NULL;
    size_t encoded_len = 0;
    WPACKET pkt;
    int ok;

    ok =   WPACKET_init_der(&pkt, buf, sizeof(buf)
        && DER_w_start_sequence(&pkt, -1)
        && DER_w_bn(&pkt, -1, bn)
        && DER_w_boolean(&pkt, 1, bool)
        && DER_w_precompiled(&pkt, -1, OID, sizeof(OID))
        && DER_w_end_sequence(&pkt, -1)
        && WPACKET_finish(&pkt)
        && WPACKET_get_total_written(&pkt, &encoded_len)
        && (p = WPACKET_get_curr(&pkt)) != NULL;

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/11450)