introduce IsPrim class containing transformWithFreeze function

This is to support scale-invariant primitives (e.g. arrowheads) but
may be useful for other stuff as well.
export IsPrim class
add IsPrim instance for Prim
change default implementation of renderDia to call transformWithFreeze
comment IsPrim class
Merge pull request #37 from diagrams/scaleinv-prim

New IsPrim class for supporting ScaleInv