Store floating point numbers in double precision

Even on 32-bit systems, Value has enough space to hold a double.

src/libexpr/value.hh

@@ -44,7 +44,7 @@ class JSONPlaceholder;
 
 
 typedef long NixInt;
-typedef float NixFloat;
+typedef double NixFloat;
 
 /* External values must descend from ExternalValueBase, so that
  * type-agnostic nix functions (e.g. showType) can be implemented

src/libutil/json.cc

@@ -31,6 +31,7 @@ template<> void toJSON<unsigned long>(std::ostream & str, const unsigned long &
 template<> void toJSON<long long>(std::ostream & str, const long long & n) { str << n; }
 template<> void toJSON<unsigned long long>(std::ostream & str, const unsigned long long & n) { str << n; }
 template<> void toJSON<float>(std::ostream & str, const float & n) { str << n; }
+template<> void toJSON<double>(std::ostream & str, const double & n) { str << n; }
 
 template<> void toJSON<std::string>(std::ostream & str, const std::string & s)
 {