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.cpp

@@ -256,7 +256,7 @@ ResultCode VMManager::ReprotectRange(VAddr target, u64 size, VMAPermission new_p
     return RESULT_SUCCESS;
 }
 
-ResultVal<VAddr> VMManager::HeapAllocate(u64 size) {
+ResultVal<VAddr> VMManager::SetHeapSize(u64 size) {
     if (size > GetHeapRegionSize()) {
         return ERR_OUT_OF_MEMORY;
     }