Store current capacity in CowData buffers, and rewrite most of it.

Add `reserve` to `CowData`, `Vector` and `Array`.

# Conflicts:
#	core/os/memory.h
#	core/templates/cowdata.h

core/variant/array.h

@@ -135,6 +135,7 @@ public:
 	_FORCE_INLINE_ void append(const Variant &p_value) { push_back(p_value); } //for python compatibility
 	void append_array(const Array &p_array);
 	Error resize(int p_new_size);
+	Error reserve(int p_new_size);
 
 	Error insert(int p_pos, const Variant &p_value);
 	void remove_at(int p_pos);