Support std::u16string message formatting

**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"
```

Start by reproducing the supplied UnicodeComparisonFormatting example with Catch2 v2.6.1 and compare its current failure output with the requested output. Done means failed CHECK and REQUIRE assertions involving std::u16string display u"str1" and u"str2" rather than numeric character values.

Written by the indexing model from the issue text.