Removed a broken Arc Fitting feature.

Removed the questionable Pressure Advance feature. It is better to use the Pressure Advance implemented into a firmware.
Added a C++ implementation of GCodeReader and SpiralVase, thanks to @alexrj
Added a C++ implementation of GCodeTimeEstimator, thanks to @lordofhyphens

xs/src/libslic3r/GCode/SpiralVase.cpp

@@ -0,0 +1,95 @@
+#include "SpiralVase.hpp"
+#include <sstream>
+
+namespace Slic3r {
+
+std::string
+_format_z(float z)
+{
+    std::ostringstream ss;
+    ss << std::fixed << std::setprecision(3)
+       << z;
+    return ss.str();
+}
+
+std::string
+SpiralVase::process_layer(const std::string &gcode)
+{
+    /*  This post-processor relies on several assumptions:
+        - all layers are processed through it, including those that are not supposed
+          to be transformed, in order to update the reader with the XY positions
+        - each call to this method includes a full layer, with a single Z move
+          at the beginning
+        - each layer is composed by suitable geometry (i.e. a single complete loop)
+        - loops were not clipped before calling this method  */
+    
+    // If we're not going to modify G-code, just feed it to the reader
+    // in order to update positions.
+    if (!this->enable) {
+        this->_reader.parse(gcode, {});
+        return gcode;
+    }
+    
+    // Get total XY length for this layer by summing all extrusion moves.
+    float total_layer_length = 0;
+    float layer_height = 0;
+    float z;
+    bool set_z = false;
+    
+    {
+        GCodeReader r = this->_reader;  // clone
+        r.parse(gcode, [&total_layer_length, &layer_height, &z, &set_z]
+            (GCodeReader &, const GCodeReader::GCodeLine &line) {
+            if (line.cmd == "G1") {
+                if (line.extruding()) {
+                    total_layer_length += line.dist_XY();
+                } else if (line.has('Z')) {
+                    layer_height += line.dist_Z();
+                    if (!set_z) {
+                        z = line.new_Z();
+                        set_z = true;
+                    }
+                }
+            }
+        });
+    }
+    
+    // Remove layer height from initial Z.
+    z -= layer_height;
+    
+    std::string new_gcode;
+    this->_reader.parse(gcode, [&new_gcode, &z, &layer_height, &total_layer_length]
+        (GCodeReader &, GCodeReader::GCodeLine line) {
+        if (line.cmd == "G1") {
+            if (line.has('Z')) {
+                // If this is the initial Z move of the layer, replace it with a
+                // (redundant) move to the last Z of previous layer.
+                line.set('Z', _format_z(z));
+                new_gcode += line.raw + '\n';
+                return;
+            } else {
+                float dist_XY = line.dist_XY();
+                if (dist_XY > 0) {
+                    // horizontal move
+                    if (line.extruding()) {
+                        z += dist_XY * layer_height / total_layer_length;
+                        line.set('Z', _format_z(z));
+                        new_gcode += line.raw + '\n';
+                    }
+                    return;
+                
+                    /*  Skip travel moves: the move to first perimeter point will
+                        cause a visible seam when loops are not aligned in XY; by skipping
+                        it we blend the first loop move in the XY plane (although the smoothness
+                        of such blend depend on how long the first segment is; maybe we should
+                        enforce some minimum length?).  */
+                }
+            }
+        }
+        new_gcode += line.raw + '\n';
+    });
+    
+    return new_gcode;
+}
+
+}