merge(yants): Integrate yants into depot at //depot/nix/yants r/248

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+yants
+=====
+
+[![Build Status](https://travis-ci.org/tazjin/yants.svg?branch=master)](https://travis-ci.org/tazjin/yants)
+
+This is a tiny type-checker for data in Nix, written in Nix.
+
+# Features
+
+* Checking of primitive types (`int`, `string` etc.)
+* Checking polymorphic types (`option`, `list`, `either`)
+* Defining & checking struct/record types
+* Defining & matching enum types
+* Defining & matching sum types
+* Defining function signatures (including curried functions)
+* Types are composable! `option string`! `list (either int (option float))`!
+* Type errors also compose!
+
+Currently lacking:
+
+* Any kind of inference
+* Convenient syntax for attribute-set function signatures
+
+## Primitives & simple polymorphism
+
+![simple](screenshots/simple.png)
+
+## Structs
+
+![structs](screenshots/structs.png)
+
+## Nested structs!
+
+![nested structs](screenshots/nested-structs.png)
+
+## Enums!
+
+![enums](screenshots/enums.png)
+
+## Functions!
+
+![functions](screenshots/functions.png)
+
+# Usage
+
+Yants can be imported from its `default.nix`. A single attribute (`lib`) can be
+passed, which will otherwise be imported from `<nixpkgs>`.
+
+Examples for the most common import methods would be:
+
+1. Import into scope with `with`:
+    ```nix
+    with (import ./default.nix {});
+    # ... Nix code that uses yants ...
+    ```
+
+2. Import as a named variable:
+    ```nix
+    let yants = import ./default.nix {};
+    in yants.string "foo" # or other uses ...
+    ````
+
+3. Overlay into `pkgs.lib`:
+    ```nix
+    # wherever you import your package set (e.g. from <nixpkgs>):
+    import <nixpkgs> {
+      overlays = [
+        (self: super: {
+          lib = super.lib // { yants = import ./default.nix { inherit (super) lib; }; };
+        })
+      ];
+    }
+
+    # yants now lives at lib.yants, besides the other library functions!
+    ```
+
+Please see my [Nix one-pager](https://github.com/tazjin/nix-1p) for more generic
+information about the Nix language and what the above constructs mean.
+
+# Stability
+
+The current API of Yants is **not yet** considered stable, but it works fine and
+should continue to do so even if used at an older version.
+
+Yants' tests use Nix versions above 2.2 - compatibility with older versions is
+not guaranteed.