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

[Teodor Dutu]

Hi,

This week I hit another dead end with regards to changing 
`_d_arrayctor`'s signature. More specifically, a signature that 
_should_ pass all tests as well as make use of NRVO is this one:
```d
Tarr1 _d_arrayctor(Tarr1 : T1[], Tarr2 : T2[], T1, T2)(scope 
Tarr2 from) @trusted
{
     // ...
     Tarr1 to;
     // ...
     return to;
}
```
However, there seems to be an bug old bug concerning nested 
structs whereby the compiler doesn't allow me to create `to` when 
`T1` is a nested struct, as in the example below, taken from 
[this](https://github.com/teodutu/druntime/blob/69ba1f900733f9929d3f704c9c66d393806a343b/src/core/internal/array/construction.d#L84-L99) unittest:
```d
@safe unittest
{
     int counter;
     struct S
     {
         int val;
         this(this) { counter++; }
     }

     S[4] arr1;
     S[4] arr2 = [S(0), S(1), S(2), S(3)];
     arr1 = _d_arrayctor!(typeof(arr1), typeof(arr2), S, 
S)(arr2[]);
}
```
I wrote 
[this](https://forum.dlang.org/post/buyuiryhdsgrnjvwisue@forum.dlang.org) forum post to ask tho community what to do about this bug. I tried to give as detailed of an account of the situation as I could. The bug is has been previously reported here, but it seems it's still active:
- https://issues.dlang.org/show_bug.cgi?id=8850
- https://issues.dlang.org/show_bug.cgi?id=8863

As a result, I've switched over to the `_d_arrayappend` hooks 
once more and am looking forward to progress with those in the 
near future.

Thanks,
Teodor