Fixes #139.

* Split colours and `pprintf(...)` into `io/pprintf.hpp` header.
* Remove generic `to_string()` function, replacing its very occasional usage with `pprintf`.
* Move block pretty printing into own .cpp file; this is the only place that the vector ostream printer was used.
* Remove `enum_hash`, as not needed with C++14.
* Move `is_in` utility function to `util.hpp`.
* Remove old SIMD printer backend code.