The `fast` option is largely broken

The fast option currently determines whether to recreate output files on the sole basis of the text form of the model equations. This approach is largely broken.

At the very least, it should check whether the options that affect the files created for the model representation have changed since the previous run (the value of those options should thus be included in the checksum). Some of those options include:

• use_dll
• mexext
• matlabroot
• C compiler path and options (either through compiler_setup or configuration file)
• mfs both dynamic and static
• cutoff both dynamic and static
• notmpterms
• transform_unary_ops
• nocommutativity
• …and probably others

It also needs to take into account steady_state_model block, and the commands that trigger parameter derivatives.