A more nuanced approach to constants

Currently there is `constants_mod` in infrastructure which holds constants used everywhere.

This is almost certainly conceptually wrong. Each model and the infrastructure should be responsible for managing their own constants. When it comes to matching type kinds this is handled by the compiler.

It is also likely to play badly with our desire to support multi-precision models.

Start by locating infrastructure/constants_mod and inventorying which constants are used by the models and infrastructure. Determine how constants related to type kinds are currently handled by the compiler, then define ownership for each remaining constant; done means the shared module no longer improperly owns model-specific constants and multi-precision concerns are addressed.

Written by the indexing model from the issue text.