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/os/memory.cpp

@@ -204,8 +204,6 @@ uint64_t Memory::get_mem_max_usage() {
 }
 
 _GlobalNil::_GlobalNil() {
-
-	color = 1;
 	left = this;
 	right = this;
 	parent = this;