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.

core/math/geometry.h

@@ -890,14 +890,14 @@ public:
 		for (int i = 0; i < n; ++i) {
 			while (k >= 2 && vec2_cross(H[k - 2], H[k - 1], P[i]) <= 0)
 				k--;
-			H[k++] = P[i];
+			H.write[k++] = P[i];
 		}
 
 		// Build upper hull
 		for (int i = n - 2, t = k + 1; i >= 0; i--) {
 			while (k >= t && vec2_cross(H[k - 2], H[k - 1], P[i]) <= 0)
 				k--;
-			H[k++] = P[i];
+			H.write[k++] = P[i];
 		}
 
 		H.resize(k);