Currently, the environment is passed as a `NValue m`, but is assumed to be
a set every single time it is used. This commit changes it `Map Text (NValue
m)`. Since this is used a lot, it defines a new type alias:
```
type ValueSet m = Map.Map Text (NValue m)
```
This has multiples benefits:
- Simplify some code by removing all the checks that the env is indeed a set
- Simplify the usage of the module by making clear that we need a set as the
environment. (I especially like this, since it took me a while to figure out
what that argument was supposed to be the first time).
- Make it simple to inject functions in the environment (for example to have
builtins) since now the function definition doesn't need to unwrap the set.
