All primitive types are unlimited.
Treat unkinded type variables as being of kind Unl, not kind Any.
Restore type signatures in prelude.links.

This has the advantage of requiring the smallest delta from existing
type signatures---if a signature worked in pre-linearity Links, it
should work now.  However, it does result in a somewhat odd behavior.
Consider the following definitions

> sig id1: (a)->a; fun id1(x){x}
> fun id2(x){x}

One might expect these to be equivalent; however, the first could not be
applied to a (non-End) channel, as the type variable 'a' is implicitly
restricted to kind 'Unl'.  In this case, the most general type signature
would be

> sig id: (a::Any)->a::Any; fun id(x){x}