Reduce unnecessary COW on Vector by make writing explicit

This commit makes operator[] on Vector const and adds a write proxy to it.  From
now on writes to Vectors need to happen through the .write proxy. So for
instance:

Vector<int> vec;
vec.push_back(10);
std::cout << vec[0] << std::endl;
vec.write[0] = 20;

Failing to use the .write proxy will cause a compilation error.

In addition COWable datatypes can now embed a CowData pointer to their data.
This means that String, CharString, and VMap no longer use or derive from
Vector.

_ALWAYS_INLINE_ and _FORCE_INLINE_ are now equivalent for debug and non-debug
builds. This is a lot faster for Vector in the editor and while running tests.
The reason why this difference used to exist is because force-inlined methods
used to give a bad debugging experience. After extensive testing with modern
compilers this is no longer the case.

modules/enet/networked_multiplayer_enet.cpp

@@ -666,7 +666,7 @@ size_t NetworkedMultiplayerENet::enet_compress(void *context, const ENetBuffer *
 	while (total) {
 		for (size_t i = 0; i < inBufferCount; i++) {
 			int to_copy = MIN(total, int(inBuffers[i].dataLength));
-			copymem(&enet->src_compressor_mem[ofs], inBuffers[i].data, to_copy);
+			copymem(&enet->src_compressor_mem.write[ofs], inBuffers[i].data, to_copy);
 			ofs += to_copy;
 			total -= to_copy;
 		}