Tidy `modcc` driver, remove optimize flag. (#404)

* Remove optimization option (use SIMD options for vectorization).
* Remove arbor utility library dependencies from modcc (pending separation of utility lib from arbor lib source).
* Split target (cpu, gpu) specification from vectorization architecture (avx2, avx512).
* Remove `Options` singleton; replace with structure local to `modcc.cpp`.
* Tidy `modcc` option parsing and main function; allow a single invocation of `modcc` to generate code for multiple backends.
* Rename generated sources to include backend target in filename.
* Always run a constant simplification pass on generated procedures.
* Remove file i/o code from `Module` and `modcc` main function; move functionality to new functions in `io` namespace. (Note: in on-going mechanism revamp, other i/o utility code will reside in the `io` namespace and subdirectory.)
* Remove classes `ConstantFolderVisitor` and `ExpressionClassifierVisitor` that are no longer used.
* Modify CMakeLists.txt files, `backends/*/fvm.cpp` to reflect the new filenames of generated sources.
* Small formatting changes in `modcc` source to reflect coding guidelines (incomplete).