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.

Userland/Libraries/LibDebug/DebugInfo.cpp

@@ -86,8 +86,8 @@ void DebugInfo::prepare_lines()
         all_lines.extend(unit.line_program().lines());
     });
 
-    HashMap<FlyString, Optional<DeprecatedString>> memoized_full_paths;
-    auto compute_full_path = [&](FlyString const& file_path) -> Optional<DeprecatedString> {
+    HashMap<DeprecatedFlyString, Optional<DeprecatedString>> memoized_full_paths;
+    auto compute_full_path = [&](DeprecatedFlyString const& file_path) -> Optional<DeprecatedString> {
         if (file_path.view().contains("Toolchain/"sv) || file_path.view().contains("libgcc"sv))
             return {};
         if (file_path.view().starts_with("./"sv) && !m_source_root.is_null())

Userland/Libraries/LibDebug/DebugInfo.h

@@ -29,7 +29,7 @@ public:
     ELF::Image const& elf() const { return m_elf; }
 
     struct SourcePosition {
-        FlyString file_path;
+        DeprecatedFlyString file_path;
         size_t line_number { 0 };
         Optional<FlatPtr> address_of_first_statement;
 

Userland/Libraries/LibDebug/Dwarf/LineProgram.cpp

@@ -138,7 +138,7 @@ void LineProgram::append_to_line_info()
     full_path.append('/');
     full_path.append(m_source_files[m_file_index].name);
 
-    m_lines.append({ m_address, FlyString { full_path.string_view() }, m_line });
+    m_lines.append({ m_address, DeprecatedFlyString { full_path.string_view() }, m_line });
 }
 
 void LineProgram::reset_registers()

Userland/Libraries/LibDebug/Dwarf/LineProgram.h

@@ -6,7 +6,7 @@
 
 #pragma once
 
-#include <AK/FlyString.h>
+#include <AK/DeprecatedFlyString.h>
 #include <AK/MemoryStream.h>
 #include <AK/Vector.h>
 #include <LibDebug/Dwarf/DwarfTypes.h>
@@ -112,20 +112,20 @@ public:
 
     struct LineInfo {
         FlatPtr address { 0 };
-        FlyString file;
+        DeprecatedFlyString file;
         size_t line { 0 };
     };
 
     Vector<LineInfo> const& lines() const { return m_lines; }
 
     struct DirectoryAndFile {
-        FlyString directory;
-        FlyString filename;
+        DeprecatedFlyString directory;
+        DeprecatedFlyString filename;
     };
     DirectoryAndFile get_directory_and_file(size_t file_index) const;
 
     struct FileEntry {
-        FlyString name;
+        DeprecatedFlyString name;
         size_t directory_index { 0 };
     };
     Vector<FileEntry> const& source_files() const { return m_source_files; }