Fix definitions of ERR in examples/l3-machine-code

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.

The ARM-M0 model is now hooked up to the "core/fast" decompiler.

Various updates/improvements to the decompiler tools.