Initial rewrite of the file accessors from Perl to C++.

This is especially important for the extremely slow AMF parser.
Also there is a new file handler for the Prusa Control 'PRUS' format.

xs/src/libslic3r/Format/PRUS.hpp

@@ -0,0 +1,14 @@
+#if defined(SLIC3R_PRUS) && ! defined(slic3r_Format_PRUS_hpp_)
+#define slic3r_Format_PRUS_hpp_
+
+namespace Slic3r {
+
+class TriangleMesh;
+class Model;
+
+// Load a PrusaControl project file into a provided model.
+extern bool load_prus(const char *path, Model *model);
+
+}; // namespace Slic3r
+
+#endif /* SLIC3R_PRUS && ! defined(slic3r_Format_PRUS_hpp_) */