kernel/vm_manager: Rename HeapAllocate to SetHeapSize

Makes it more obvious that this function is intending to stand in for
the actual supervisor call itself, and not acting as a general heap
allocation function.

Also the following change will merge the freeing behavior of HeapFree
into this function, so leaving it as HeapAllocate would be misleading.

src/core/hle/kernel/vm_manager.h

@@ -380,7 +380,7 @@ public:
     /// Changes the permissions of a range of addresses, splitting VMAs as necessary.
     ResultCode ReprotectRange(VAddr target, u64 size, VMAPermission new_perms);
 
-    ResultVal<VAddr> HeapAllocate(u64 size);
+    ResultVal<VAddr> SetHeapSize(u64 size);
     ResultCode HeapFree(VAddr target, u64 size);
 
     ResultCode MirrorMemory(VAddr dst_addr, VAddr src_addr, u64 size, MemoryState state);