Modify the type of HolKernel.syntax_fns and add some documentation.

This is in response to issue #247.

The change to the type makes things a little more natural when using partial applications - for example, HolKernel.syntax_fns1 can be used to provide syntax support for regular unary operations.

Support for generating TEMPORAL triples.

Currently works for ARM, M0 and MIPS models. Yet to update the x86-64 model.