Replace Value type setters with mk* functions

Move clearValue inside Value

mkInt instead of setInt

mkBool instead of setBool

mkString instead of setString

mkPath instead of setPath

mkNull instead of setNull

mkAttrs instead of setAttrs

mkList instead of setList*

mkThunk instead of setThunk

mkApp instead of setApp

mkLambda instead of setLambda

mkBlackhole instead of setBlackhole

mkPrimOp instead of setPrimOp

mkPrimOpApp instead of setPrimOpApp

mkExternal instead of setExternal

mkFloat instead of setFloat

Add note that the static mk* function should be removed eventually

src/libexpr/nixexpr.hh

@@ -129,7 +129,7 @@ struct ExprPath : Expr
 {
     string s;
     Value v;
-    ExprPath(const string & s) : s(s) { mkPathNoCopy(v, this->s.c_str()); };
+    ExprPath(const string & s) : s(s) { v.mkPath(this->s.c_str()); };
     COMMON_METHODS
     Value * maybeThunk(EvalState & state, Env & env);
 };