Optimize frontend+backend object instantiation

Currently the stats obtained via dtrace for all our tests, i.e. using dtrace on TestRunner, are the following:

```
Block::shrd_ctor_copy 1977
DataArray::shrd_ctor_copy 1123
DataTag::shrd_ctor_copy 71
Dimension[Range]::shrd_ctor_copy 15
Dimension[Set]::shrd_ctor_copy 14
File::shrd_ctor_copy 6737
ImplContainer::ctor_copy 998
ImplContainer::ctor_empty 13554
ImplContainer::ctor_total 28381
ImplContainer::shrd_ctor_copy 13829
ImplContainer::shrd_ctor_total 13829
Property::shrd_ctor_copy 102
Section::shrd_ctor_copy 1888
SimpleTag::shrd_ctor_copy 77
Source::shrd_ctor_copy 1822
```

This means in total we create 28381 `ImplContainer` based objects, 13554 of them are uninitialised, i.e. created via the constructor that takes no args (`ctor_empty`), and 13829 objects are initialised via the constructor that takes the a const reference to the `shared_ptr`, which then in turn copy-constructs the to-be-constructed-`ImplContainer`'s `shared_ptr`.
I very much suspect that we do some unnecessary copies since all of our current objects lack any move operation support.