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`.

The previous code used some ad-hoc attribute enumeration to check for missing
fields and did not really type check the attributes, because it used `t.check`
from the module system types, which only does shallow checks for nested types,
e.g. just `builtins.isList` for (listOf strings).

This new version uses `types-simple` to recursively check every attribute,
outputting accurate information for type errors in nested fields, e.g.:

Package ‘hello-2.10’ in … has an invalid meta attrset:
maintainers.1.email should be: string
but is: null

When we set
meta.maintainers [ eelco foobar ];
in `hello/default.nix` and foobar is defined as
{ name = "foobar"; email = null; };
in `maintainers-list.nix`.

The path accurately pins down where the type check failed:
In the `maintainers` field, the second element of the list (counting from 0)
and the field email in that element.

Further work: If an unsupported meta field is used, the type error will print
the complete type of the meta product, which is quite big. The type error should
show a diff of the problematic fields in that case.
This is possible but not yet integrated into the type checking logic.

The meta type is changed to either a string, a license or a list of licenses,
where license is an attrset with required `fullName`, `shortName` and `free`
fields, and two optional fields.

Every package that does not conform to this union type is fixed; I noticed that
only a few packages used `list of strings`, so I corrected them and removed the
alternative.

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attributes shallowly, but that has been replaced by `productOpt`, so we don’t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.

`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.