AK+Everywhere: Rename FlyString to DeprecatedFlyString

DeprecatedFlyString relies heavily on DeprecatedString's StringImpl, so
let's rename it to A) match the name of DeprecatedString, B) write a new
FlyString class that is tied to String.

AK/CMakeLists.txt

@@ -2,9 +2,9 @@ set(AK_SOURCES
     Assertions.cpp
     Base64.cpp
     CircularBuffer.cpp
+    DeprecatedFlyString.cpp
     DeprecatedString.cpp
     FloatingPointStringConversions.cpp
-    FlyString.cpp
     Format.cpp
     FuzzyMatch.cpp
     GenericLexer.cpp