AsTransformation

Built on demand, and it has to be. RewritingTransformation derives
from Transformation, so constructing one runs that type's static
initialiser -- which reads RewriteRules. Doing it in this
constructor instead would make the two types depend on each other's
initialisation and hand whichever one lost the race a null rule set. Two threads
arriving together may each build one; they are equivalent and immutable, so
whichever reference lands is the one everyone then uses.