Adjust XML allocation pages to have the exact specified size

Previously the page size was defining the data size, and due to additional
headers (+ recently removed allocation padding) the actual allocation was a bit
bigger.

The problem is that some allocators round 2^N+k allocations to 2^N+M, which can
result in noticeable waste of space. Specifically, on 64-bit OSX allocating the
previous page size (32k+40) resulted in 32k+512 allocation, thereby wasting 472
bytes, or 1.4%.

Now we have the allocation size specified exactly and just recompute the available
data size, which can in small space savings depending on the allocator.

src/pugixml.cpp

@@ -403,14 +403,6 @@ PUGI__NS_END
 #endif
 
 PUGI__NS_BEGIN
-	static const size_t xml_memory_page_size =
-	#ifdef PUGIXML_MEMORY_PAGE_SIZE
-		PUGIXML_MEMORY_PAGE_SIZE
-	#else
-		32768
-	#endif
-		;
-
 #ifdef PUGIXML_COMPACT
 	static const uintptr_t xml_memory_block_alignment = 4;
 #else
@@ -476,6 +468,14 @@ PUGI__NS_BEGIN
 	#endif
 	};
 
+	static const size_t xml_memory_page_size =
+	#ifdef PUGIXML_MEMORY_PAGE_SIZE
+		(PUGIXML_MEMORY_PAGE_SIZE)
+	#else
+		32768
+	#endif
+		- sizeof(xml_memory_page);
+
 	struct xml_memory_string_header
 	{
 		uint16_t page_offset; // offset from page->data