AK: Use size_t for the length of strings

Using int was a mistake. This patch changes String, StringImpl,
StringView and StringBuilder to use size_t instead of int for lengths.
Obviously a lot of code needs to change as a result of this.

Kernel/FileSystem/FileSystem.cpp

@@ -46,7 +46,7 @@ FS::DirectoryEntry::DirectoryEntry(const char* n, InodeIdentifier i, u8 ft)
     name[name_length] = '\0';
 }
 
-FS::DirectoryEntry::DirectoryEntry(const char* n, int nl, InodeIdentifier i, u8 ft)
+FS::DirectoryEntry::DirectoryEntry(const char* n, size_t nl, InodeIdentifier i, u8 ft)
     : name_length(nl)
     , inode(i)
     , file_type(ft)