implement rule DCL16-C (trivial)
partially implement rule DCL31-C (trivial)
rewrite testing infrastructure

now identifying problems only(!) to be found with extra analysis
extend DCL31-C for completeness
add isTrue function to conditional lib

converts Choice[Boolean] into FeatureExpr
implement DCL36-C. Do not declare an identifier with conflicting linkage classifications

requires to add linkage information to the symbol table, which is quite a crosscutting change
adjust command-line parser to support new analyses
additional test of existing behavior
add severityExtra to error messages