Port member initialization from constructor to declaration (C++11)

Using `clang-tidy`'s `modernize-use-default-member-init` check and
manual review of the changes, and some extra manual changes that
`clang-tidy` failed to do.

Also went manually through all of `core` to find occurrences that
`clang-tidy` couldn't handle, especially all initializations done
in a constructor without using initializer lists.

core/io/stream_peer.cpp

@@ -536,8 +536,3 @@ Ref<StreamPeerBuffer> StreamPeerBuffer::duplicate() const {
 	spb->data = data;
 	return spb;
 }
-
-StreamPeerBuffer::StreamPeerBuffer() {
-
-	pointer = 0;
-}