Improve TransformKey to pack more kinds of transforms efficiently

The bespoke caching scheme in LambdaFormEditor.TransformKey allows keys to be very compact when all byte values fit in 4 bits, otherwise a byte array is allocated and used. This means that all transforms with a kind value above 15 will be forced to allocate and use array comparisons.

Removing unused and folding some transforms to ensure all existing kinds can fit snugly within the 0-15 value range realize a minor improvement to footprint, speed and allocation pressure of affected transforms, e.g. ~300bytes/op reduction in the StringConcatFactoryBootstraps microbenchmark