Avoid holding entire file in memory

Previously we need a file buffer that is as large as the file size and
this could be a problem when user has less free memory available or
having very large data. Now with the help of `FileOutputIterator`,
we can have a much smaller, fixed size immediate file buffer and also
the code looks nice with `lt::bencode()`.

src/base/CMakeLists.txt

@@ -58,6 +58,7 @@ utils/bytearray.h
 utils/foreignapps.h
 utils/fs.h
 utils/gzip.h
+utils/io.h
 utils/misc.h
 utils/net.h
 utils/password.h
@@ -133,6 +134,7 @@ utils/bytearray.cpp
 utils/foreignapps.cpp
 utils/fs.cpp
 utils/gzip.cpp
+utils/io.cpp
 utils/misc.cpp
 utils/net.cpp
 utils/password.cpp