Hierarchical builtins

[Gbury]

This PR adds some hierarchy in the typed builtins. Previously all the builtins lived in a flat namespace, but now there is (most of the time) one extensible constructor builtin by theory, which wraps a dedicated (regular) type for the builtins of that theory. This brings a few benefits:

• We can have an exhaustivity check for functions operating on the primitives of one theory
• we can have more freedom in the naming of builtins, since we have different namespaces for the builtins of each theory
• This may result in a slight speedup, since matching on extensible variants is not very optimized

Note that this is a rather large breaking change, since it will likely require to update any pattern matching on builtins, but the translation should be rather mechanical. It is not completely mechanical because I took the opportunity to rename some builtins, taking advantage of the now different namespaces for builtins of different theories (and also to avoid repetitions like | Builtin.Float Fp_foo -> ...).

cc @bobot , in particular for the new names of primitives in the Float theory.

[bobot]

Just typos, thanks!