kernel+libos+libluna: Avoid copying and reallocation when creating Strings

This is done by reusing the existing buffers in Vector and Buffer instances.

kernel/src/memory/MemoryManager.cpp

@@ -431,7 +431,7 @@ namespace MemoryManager
 
         TRY(result.try_append(0)); // null terminator
 
-        return String::from_cstring(result.data());
+        return String { result.release_data() };
     }
 
     bool validate_access(const void* mem, usize size, int flags)

libluna/src/Base64.cpp

@@ -152,6 +152,6 @@ namespace Base64
         u8 nul_byte = '\0';
         TRY(buf.append_data(&nul_byte, 1));
 
-        return String::from_cstring((char*)buf.data());
+        return String { (char*)buf.release_data() };
     }
 }

libluna/src/Buffer.cpp

@@ -52,3 +52,10 @@ Result<void> Buffer::append_data(const u8* data, usize size)
 
     return {};
 }
+
+u8* Buffer::release_data()
+{
+    u8* data = m_data;
+    m_data = nullptr;
+    return data;
+}

libluna/src/String.cpp

@@ -140,7 +140,7 @@ Result<String> String::vformat(StringView fmt, va_list ap)
 
     TRY(buf.try_append(0));
 
-    return from_cstring(buf.data());
+    return String { buf.release_data() };
 }
 
 Result<String> String::from_cstring(const char* str)

libluna/src/StringBuilder.cpp

@@ -61,5 +61,5 @@ Result<String> StringBuilder::string()
     char nul = '\0';
     TRY(m_data.append_data((u8*)&nul, 1));
 
-    return String::from_cstring((const char*)m_data.data());
+    return String { (char*)m_data.release_data() };
 }

libos/src/File.cpp

@@ -143,7 +143,7 @@ namespace os
 
         TRY(data.try_append('\0'));
 
-        return String::from_cstring(data.data());
+        return String { data.release_data() };
     }
 
     Result<String> File::read_all_as_string()