Support std::u16string message formatting

[grhoten]

Description
It would be nice if UTF-16 strings were converted to UTF-8 strings automatically for when reporting failures. If you're working with UTF-16 strings frequently, it would make it easier and faster if I didn't have to prefix these failures with INFO lines that convert std::u16string to std::string to actually see what these string values actually were.

Additional context
Here is a sample test case that uses catch.hpp version 2.6.1. It requires C++11 or later to work.

TEST_CASE("UnicodeComparisonFormatting")
{
    ::std::u16string str1(u"str1");
    ::std::u16string str2(u"str2");
    CHECK(str1 == str2);
    REQUIRE(str1 == str2);
}

Here is the current output. I changed the path to ... for brevity.

...UnicodeComparisonFormatting.cpp:13: FAILED:
  CHECK( str1 == str2 )
with expansion:
  { 115, 116, 114, 49 }
  ==
  { 115, 116, 114, 50 }

...UnicodeComparisonFormatting.cpp:14: FAILED:
  REQUIRE( str1 == str2 )
with expansion:
  { 115, 116, 114, 49 }
  ==
  { 115, 116, 114, 50 }

Here is the desired output with the exact same code

...UnicodeComparisonFormatting.cpp:13: FAILED:
  CHECK( str1 == str2 )
with expansion:
  u"str1" == u"str2"

...UnicodeComparisonFormatting.cpp:14: FAILED:
  REQUIRE( str1 == str2 )
with expansion:
  u"str1" == u"str2"