Refactor/mut

[leissa]

As I've already pointed out several times the name nominal is confusing as nominals are in fact not nominal as they are tested like structurals for alpha-equiv.

This patch renames:

• nominal -> mutable
• structural -> immutable

In the future I'd like to add the option to make mutables truely nominal/unique.

In addition: Some more porting from const Def* -> Ref.

[NeuralCoder3]

In addition: Some more porting from const Def* -> Ref.

Is there an easy rule for when to use Ref and when to use Def (const Def*)? E.g. should a plugin developer

• always use const Def*
• always use Ref
• use const Def* except for normalizers?

[leissa]

In addition: Some more porting from const Def* -> Ref.

Is there an easy rule for when to use Ref and when to use Def (const Def*)? E.g. should a plugin developer

* always use `const Def*`

* always use `Ref`

* use `const Def*` except for normalizers?

The rule is to always use Ref instead of const Def*. For everything else such as Def* or const App* or Sigma*, use these types instead.