Merge #970: Changes to Shorthands (& op precedence); ParserTests refactor

Walked into the `Shorthands` module.

Main things: https://github.com/haskell-nix/hnix/blob/2021-07-09-changes/ChangeLog.md
  * fixed `mkAssert`
  * refactored `ParserTests`.
  * Added `@.<|>` operator that represents (previously unrepresentable by shorthands) feature of a language: `s.x or y`.
  * Established precedence between shorthand operators. This allows to write down the Haskell (pretty) directly to represent a one-to-one a Nix expression.

Author: Anton-Latukha

ChangeLog.md

@@ -1,7 +1,37 @@
 
 # ChangeLog
 
-## [(diff)](https://github.com/haskell-nix/hnix/compare/0.13.1...0.14.0#files_bucket) 0.14.0
+## [(diff)](https://github.com/haskell-nix/hnix/compare/0.14.0...0.15.0#files_bucket) 0.15.0
+
+Partial log (for now):
+
+* Breaking:
+
+  * `Nix.Expr.Shorthands`:
+    * `inherit{,From}`: dropped second argument as irrelevant ([report](https://github.com/haskell-nix/hnix/issues/326)).
+    * `mkAssert`: fixed ([report](https://github.com/haskell-nix/hnix/issues/969)).
+    * fx presedence between the operators:
+        
+        ```haskell
+        (@@), (@.), (@./), ($==), ($!=), ($<), ($<=), ($>), ($>=), ($&&), ($||), ($->), ($//), ($+), ($-), ($*), ($/), ($++), (==>)
+        ```
+        
+        Now these shorthands can be used without sectioning & so represent the Nix expressions one to one.
+        
+        ```haskell
+        nix = "           a/b   //            c/def   //           <g>  <             def/d"
+        hask = mkRelPath "a/b" $// mkRelPath "c/def" $// mkEnvPath "g" $<  mkRelPath "def/d"
+        ```
+
+* Additional
+  * `Nix.Expr.Shorthands`:
+    * `mkOper{,2}` entered deprecation, superceeded by new name `mkOp{,2}`.
+    * `mkBinop` entered deprecation, supeceeded by new name `mkBinop`.
+    * added `@.<|>` for Nix language `s.x or y` expession.
+    * add `mkNeg` number negation.
+
+
+## [(diff)](https://github.com/haskell-nix/hnix/compare/0.13.1...0.14.0#files_bucket) 0.14.0 (2021-07-08)
 
 * GHC 9.0 support.
 

src/Nix/Builtins.hs

@@ -583,7 +583,7 @@ headNix :: forall e t f m. MonadNix e t f m => NValue t f m -> m (NValue t f m)
 headNix =
   maybe
     (throwError $ ErrorCall "builtins.head: empty list")
-    (pure)
+    pure
   . viaNonEmpty head <=< fromValue @[NValue t f m]
 
 tailNix :: forall e t f m. MonadNix e t f m => NValue t f m -> m (NValue t f m)