is int and is set constraints don't check their type #10

Problem

The is int and is set constraints don't check what they say. In ConstraintDescriptor.check:

• is int only checks that the field is a single value, not that it's an integer.
• is set only checks that the field isn't a list. A plain value counts as unordered, so it passes.

Tested against v0.1.2:

S subset int does check for integers correctly (it uses int.TryParse), so is int can reuse that check.

Fix

• is int: require a value that parses as an integer.
• is set: require an unordered collection (not a value).
• is list: likewise require an ordered collection. Check it, since it has the same structure.
• Throw the dedicated parse exception from Clear parse errors #4 on failure.

Redux context

Redux grammars rely on these, e.g. w is int for edge weights in MAXCUT and N is set in Partition, so bad input can get past the grammar today. Related to the type checks in #6.