For consistency, TestParser renamed to TestSystemParser
Verify partial parsing
Parsing with presence
Oops. No harm, though.
Found this infinite-loop problem by inspection
Implement and test SystemParser::whichObject
ObjectPtr is not necessary to parsing; only IDs
Make the sender an argument to SystemObject::send

Also drop ABBR as a message; it was never used.
Basic ‘include’ statement working

Not tested with files yet, only string sources.

Added Wellspring, ‘null’ and ‘system’ sentinels to Universe, and introduced SourceFilePtr
Minor usage string update