[SAoC 2021] Replace druntime Hooks with Templates: Milestone 3, Week 2

[Teodor Dutu]

Hi,

This week I ran a benchmark for the 3 possible approaches to 
`_d_arrayctor`. They are: the old hook on one hand and two 
implementations of the new template. One of these uses a third 
pointer parameter for `_d_arractor`, in order to convert its 
strong purity to weak purity, while the other only takes the 
source array as a parameter and returns the created array. The 
latter doesn't use NRVO, thought. I gave more details about these 
3 approaches and abount my benchmark in [this post].

After finishing the benchmark, I resumed work on 
`_d_arrayappendcTX`. I'm currently debugging an error where the 
template is called 3 times instead of one and I'm trying to 
figure out what code in the compiler is re-run so that this ends 
up happening.

If I get stuck with `_d_arrayappendcTX`, I'll start looking at 
`_d_newitem{T,iT,U}`. These hooks are likely to be simpler to 
implement because they only call the GC.

Thanks,
Teodor