Rename fix.v → fixpoint.v because fix is a reserved keyword.

That caused some problems, e.g.:

  From iris.base_logic Require Export fix.

Gave:

  Syntax error: [constr:global] expected after
  [export_token] (in [vernac:gallina_ext]).

_CoqProject

@@ -34,7 +34,7 @@ theories/base_logic/hlist.v
 theories/base_logic/soundness.v
 theories/base_logic/double_negation.v
 theories/base_logic/deprecated.v
-theories/base_logic/fix.v
+theories/base_logic/fixpoint.v
 theories/base_logic/lib/iprop.v
 theories/base_logic/lib/own.v
 theories/base_logic/lib/saved_prop.v