asn1: use LTC_SET_ASN1 macro to access ltc_asn1_list elements
fix constructed type identifier check
bypass constructed type identification for sets / sequences
unwind conditional expressions

I prefer readability
add missing types to decode routines
change the ASN1 type to be a typedef

replace all 'default' cases in the switch statements
add missing check of the OID
trim trailing spaces
add missing ASN1 types in der_encode_set()
use DO() macro when calling API functions in tests
Merge branch 'fix/asn1' into develop

This closes #49

[skip ci]