Use std::string_view and fmtlib across all libraries (#3402)

- Twine, StringRef, Format, and NativeFormatting have been removed
- Logging now uses fmtlib style formatting
- Nearly all uses of wpi::outs/errs have been replaced with fmt::print() or
std::puts()/std::fputs() (for unformatted strings).
- A wpi/fmt/raw_ostream.h header has been added to enable
fmt::print() with wpi::raw_ostream

wpiutil/src/test/native/cpp/json/unit-unicode.cpp

@@ -37,7 +37,7 @@ SOFTWARE.
 #include "unit-json.h"
 using wpi::json;
 
-#include "wpi/Format.h"
+#include "fmt/format.h"
 #include "wpi/StringExtras.h"
 #include "wpi/raw_ostream.h"
 
@@ -919,11 +919,7 @@ static std::string codepoint_to_unicode(std::size_t cp)
     // reverse solidus, followed by the lowercase letter u, followed
     // by four hexadecimal digits that encode the character's code
     // point
-    std::string s;
-    wpi::raw_string_ostream ss(s);
-    ss << "\\u" << wpi::format_hex_no_prefix(cp, 4);
-    ss.flush();
-    return s;
+    return fmt::format("\\u{:04x}", cp);
 }
 
 // correct sequences