Document stream: truncate final unfinished document and give access to the number of truncated bytes. (#1534)

* Truncate final unclosed string.

* Adding more precise remarks.

* Better documentation and more robust code.

* ARM + PPC corrections.

* Patching ARM implementation with new stage1_mode parameter.

* Fixed most problems.

* Correcting white spaces and adding a remark.

* This adds the truncated_bytes() method to the stream instances.

tests/document_tests.cpp

@@ -222,7 +222,7 @@ int main(int argc, char *argv[]) {
   }
   // We want to know what we are testing.
   std::cout << "Running tests against this implementation: " << simdjson::active_implementation->name();
-  std::cout << "(" << simdjson::active_implementation->description() << ")" << std::endl;
+  std::cout << " (" << simdjson::active_implementation->description() << ")" << std::endl;
   std::cout << "------------------------------------------------------------" << std::endl;
 
   std::cout << "Running document tests." << std::endl;