Check UTF-8 encoding, and convert strings to UTF-8

Handle conversion errors properly by inserting � instead.  Add a binary
header to provide portable way to write binary literals (I like them) Also
add a test file.  ref #2404

src/test.cpp

@@ -0,0 +1,43 @@
+/**
+ * Just a very simple test suite, by hand, using assert()
+ */
+
+#include <assert.h>
+
+#include <iostream>
+
+#include <utils/encoding.hpp>
+#include <string.h>
+
+#include <fstream>
+
+int main()
+{
+  /**
+   * Encoding
+   */
+  const char* valid = "C̡͔͕̩͙̽ͫ̈́ͥ̿̆ͧ̚r̸̩̘͍̻͖̆͆͛͊̉̕͡o͇͈̳̤̱̊̈͢q̻͍̦̮͕ͥͬͬ̽ͭ͌̾ͅǔ͉͕͇͚̙͉̭͉̇̽ȇ͈̮̼͍͔ͣ͊͞͝ͅ ͫ̾ͪ̓ͥ̆̋̔҉̢̦̠͈͔̖̲̯̦ụ̶̯͐̃̋ͮ͆͝n̬̱̭͇̻̱̰̖̤̏͛̏̿̑͟ë́͐҉̸̥̪͕̹̻̙͉̰ ̹̼̱̦̥ͩ͑̈́͑͝ͅt͍̥͈̹̝ͣ̃̔̈̔ͧ̕͝ḙ̸̖̟̙͙ͪ͢ų̯̞̼̲͓̻̞͛̃̀́b̮̰̗̩̰̊̆͗̾̎̆ͯ͌͝.̗̙͎̦ͫ̈́ͥ͌̈̓ͬ";
+  assert(utils::is_valid_utf8(valid) == true);
+  const char* invalid = "\xF0\x0F";
+  assert(utils::is_valid_utf8(invalid) == false);
+  const char* invalid2 = "\xFE\xFE\xFF\xFF";
+  assert(utils::is_valid_utf8(invalid2) == false);
+
+  std::string in = "coucou les copains  ♥ ";
+  assert(utils::is_valid_utf8(in.c_str()) == true);
+  std::string res = utils::convert_to_utf8(in, "UTF-8");
+  assert(utils::is_valid_utf8(res.c_str()) == true && res == in);
+
+  std::string original_utf8("couc¥ou");
+  std::string original_latin1("couc\xa5ou");
+
+  // When converting back to utf-8
+  std::string from_latin1 = utils::convert_to_utf8(original_latin1.c_str(), "ISO-8859-1");
+  assert(from_latin1 == original_utf8);
+
+  // Check the behaviour when the decoding fails (here because we provide a
+  // wrong charset)
+  std::string from_ascii = utils::convert_to_utf8(original_latin1, "US-ASCII");
+  assert(from_ascii == "couc�ou");
+  return 0;
+}