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/GCodeReader.cpp

@@ -0,0 +1,109 @@
+#include "GCodeReader.hpp"
+#include <boost/algorithm/string/classification.hpp>
+#include <boost/algorithm/string/split.hpp>
+#include <fstream>
+#include <iostream>
+
+namespace Slic3r {
+
+void
+GCodeReader::apply_config(const PrintConfigBase &config)
+{
+    this->_config.apply(config, true);
+    this->_extrusion_axis = this->_config.get_extrusion_axis()[0];
+}
+
+void
+GCodeReader::parse(const std::string &gcode, callback_t callback)
+{
+    std::istringstream ss(gcode);
+    std::string line;
+    while (std::getline(ss, line))
+        this->parse_line(line, callback);
+}
+
+void
+GCodeReader::parse_line(std::string line, callback_t callback)
+{
+    GCodeLine gline(this);
+    gline.raw = line;
+    if (this->verbose)
+        std::cout << line << std::endl;
+    
+    // strip comment
+    {
+        size_t pos = line.find(';');
+        if (pos != std::string::npos) {
+            gline.comment = line.substr(pos+1);
+            line.erase(pos);
+        }
+    }
+    
+    // command and args
+    {
+        std::vector<std::string> args;
+        boost::split(args, line, boost::is_any_of(" "));
+        
+        // first one is cmd
+        gline.cmd = args.front();
+        args.erase(args.begin());
+        
+        for (std::string &arg : args) {
+            if (arg.size() < 2) continue;
+            gline.args.insert(std::make_pair(arg[0], arg.substr(1)));
+        }
+    }
+    
+    // convert extrusion axis
+    if (this->_extrusion_axis != 'E') {
+        const auto it = gline.args.find(this->_extrusion_axis);
+        if (it != gline.args.end()) {
+            std::swap(gline.args['E'], it->second);
+            gline.args.erase(it);
+        }
+    }
+    
+    if (gline.has('E') && this->_config.use_relative_e_distances)
+        this->E = 0;
+    
+    if (callback) callback(*this, gline);
+    
+    // update coordinates
+    if (gline.cmd == "G0" || gline.cmd == "G1" || gline.cmd == "G92") {
+        this->X = gline.new_X();
+        this->Y = gline.new_Y();
+        this->Z = gline.new_Z();
+        this->E = gline.new_E();
+        this->F = gline.new_F();
+    }
+}
+
+void
+GCodeReader::parse_file(const std::string &file, callback_t callback)
+{
+    std::ifstream f(file);
+    std::string line;
+    while (std::getline(f, line))
+        this->parse_line(line, callback);
+}
+
+void
+GCodeReader::GCodeLine::set(char arg, std::string value)
+{
+    const std::string space(" ");
+    if (this->has(arg)) {
+        size_t pos = this->raw.find(space + arg)+2;
+        size_t end = this->raw.find(' ', pos+1);
+        this->raw = this->raw.replace(pos, end-pos, value);
+    } else {
+        size_t pos = this->raw.find(' ');
+        if (pos == std::string::npos) {
+            this->raw += space + arg + value;
+        } else {
+            this->raw = this->raw.replace(pos, 0, space + arg + value);
+        }
+    }
+    this->args[arg] = value;
+}
+
+}