'Holes' need arbitrary extra modifiers and constraints

[alimpfard]

filtering only on a single message might not be enough

needs:

• decent syntax
• decently fast impl for specific csp

initial syntax ideas:

• ?<Constraint, Constraint> - needs a bit of work on both the parser and the lexer
• leave it to a userspace metafunction to implement (@comptime ?{Constraint}) and just add some syntactic sugar for it

[alimpfard]

temporarily resolved:

syntax: (? ?: constraint ?: constraint)
constraints can only be responds_to checks

[alimpfard]

constraints can now also be link and equality checks (not if they're strings), same syntax