A simple type system to check plain nix values.
Please see the module comments for further explanations.

Will be used to correct the buggy meta-field checks and help in sanity-checking
other parts of nixpkgs.

productOpt: a product type with the possibility for optional fields
defaults: generate (semi-arbitrary) default values for a given type

In preparation for the rewrite of `check-meta.nix`.

Especially document the inner workings of the `checkType` function.

`defaults` originally only was a crutch to make products accept optional
attributes shallowly, but that has been replaced by `productOpt`, so we don’t
need it anymore.

Also a few other changes based on input by @aszlig.

`restrict` is able to restrict the inhabitants of a type, meaning only a subset
of a type is allowed, the new type is “smaller”, but still gives nice type
errors if a nested value has the wrong type.

See the tests for some usage examples.

Open products can have additional fields in the actual value, which always
succeed in the type check (effectively default to `any`).

When defining a `productOpt` type, `req` and `opt` must not contain the same
fields, otherwise the type checking behaviour is undefined.

This can be used to check the fields of derivations (according to the
descriptions in the nix manual) and will also recurse into the meta fields.

`names` was originally intended to be able to match on the specific instance of
a type, but it breaks the abstraction in a way.
Since the function that used it was removed, we can remove the name attributes
as well.