Replace druntime Hooks with Templates: Milestone 1, Week 2

[Teodor Dutu]

Hi,

This week I faced a few setbacks with `_d_arraycatnTX` due to 
type mismatches.
Due to the hustle of moving house, I haven't made much progress 
on this front.

To take a break from this, I temporarily moved on to the 
`_d_newitem{T,iT,U}` hooks.
I implemented templated one templated hook for `_d_newitemT` and 
added unittests for it.
The difference will now be made by the compiler during the 
lowering, by calling the constructor, copying the default 
initialiser, or doing nothing (`void` initialisation).

This week I am looking to make some progress with this hook and 
come up with the aforementioned lowerings.

Thanks,\
Teodor