ASN1.y: improved detection of built-in types

The built-in types are now detected via the yacc syntax and not in the _asn1_yyerror() routine. This addresses a memory leak on invalid syntax, and also reduces the generated tree by eliminating unnecessary types.

The existing test-suite is sufficient for this change.

Resolves: #16 (closed)

Checklist

• Code modified for feature
• Test suite updated with functionality tests
• Test suite updated with negative tests
• Documentation updated

Reviewer's checklist:

• There is a test suite reasonably covering new functionality or modifications
• Function naming, parameters, return values, types, etc., are consistent with other code
• This feature/change has adequate documentation added
• No obvious mistakes in the code