Add an Opcode::constraints() method which returns an OpcodeConstraints object.
This object provides information on instruction polymorphism and how many
results is produced.
Generate a list of TypeSet objects for checking free type variables. The type
sets are parametrized rather than being represented as fully general sets.
Add UniqueTable and UniqueSeqTable classes to the meta code generator. Use for
compressing tabular data by removing duplicates.
Don't use assertions to enforce the limits on SIMD lanes in a type, Type is too
fundamental for that. Instead, the Vector-forming by() method returns an
Optional<Type>, and None if the requested SIMD vector is not valid.
Same for Type::half_vector().
