Fixes#142404
The parser can't tell the difference between array indexing and a
substring: that has to be done in semantics once we have types.
Substrings can only be in the form string([lower]:[higher]) not
string(index) or string(lower:higher:step). I added semantic checks to
catch this for the DEPEND clause.
This patch also adds lowering for correct substrings and for complex
part references.
4a47634a00