Fix -Wsubobject-linkage warning (#6243)

* Fix -Wsubobject-linkage warning

Having tk::spline header-only implementation included from
SmallAreaInfillFlowCompensator.hpp makes
SmallAreaInfillFlowCompensator::flowModel have separate (albeit the
same) implementation in each translation unit.

In order to fix this issue, SmallAreaInfillFlowCompensator::flowModel
converted to opaque 'pimpl'

* spline: remove anonymous namespace

Remove outer anonymous namespace from splice.h to make
forward declaration for tk::spline possible.

src/spline/spline.h

@@ -46,11 +46,6 @@
 #pragma GCC diagnostic ignored "-Wunused-function"
 #endif
 
-// unnamed namespace only because the implementation is in this
-// header file and we don't want to export symbols to the obj files
-namespace
-{
-
 namespace tk
 {
 
@@ -942,8 +937,6 @@ std::vector<double> solve_cubic(double a, double b, double c, double d,
 } // namespace tk
 
 
-} // namespace
-
 #if !defined(_MSC_VER)
 #pragma GCC diagnostic pop
 #endif