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/base.pri

@@ -73,6 +73,7 @@ HEADERS += \
     $$PWD/utils/foreignapps.h \
     $$PWD/utils/fs.h \
     $$PWD/utils/gzip.h \
+    $$PWD/utils/io.h \
     $$PWD/utils/misc.h \
     $$PWD/utils/net.h \
     $$PWD/utils/password.h \
@@ -143,6 +144,7 @@ SOURCES += \
     $$PWD/utils/foreignapps.cpp \
     $$PWD/utils/fs.cpp \
     $$PWD/utils/gzip.cpp \
+    $$PWD/utils/io.cpp \
     $$PWD/utils/misc.cpp \
     $$PWD/utils/net.cpp \
     $$PWD/utils/password.cpp \