[SAoC 2021] Replace druntime Hooks with Templates: Milestone 4, Week 1

[Teodor Dutu]

Hi,

This milestone I am planning to finish replacing the hooks to 
`_d_arrayappend{T,cTX}` with templates. This week I almost 
finished this task.
There is still a bug when inlining a function such as:
```d
int[] arr;
void foo()
{
	(arr ~= 2) ~= 1;
}
```
The first concatenation causes an `ArrayIndexError` because the 
location of the array length is incorrectly referred in the 
resulting executable. I am yet to find the source of this error. 
I dissected inline.d, and the resulting statement looks alright.

After this, I plan to convert `_d_newThrowable` and 
`_d_newitem{U,iT,T}` to templates. I started working on 
`_d_newThrowable`, but I'm having trouble recreating [these 
`assert`s](https://github.com/dlang/druntime/blob/fd9a45448244fb9dd4326520ad8526c540895eb0/src/rt/ehalloc.d#L35-L36) as the template doesn't store those flags.

Merry Christmas!

Thanks,
Teodor