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;