Add function asn1_parser2tree_mem()

Description of problem:

Currently we have asn1_parser2tree() which takes input from a given filename. This requires _asn1_yylex() to do lot's of (buffered but slow) I/O. A function that takes a block of memory seems to be more elegant, less code and faster execution.

Also, the current situation forces us to 'overwrite' fopen() when fuzzing asn1_parser2tree(). That introduces non-portable functions like dlsym() and fmemopen(). Regression testing the fuzz corpora as we do with make check has the same caveats. Thus the tests can't be run on every platform.