[SAoC 2021] Replace druntime Hooks with Templates: Milestone 1, Week 2

[Teodor Dutu]

Hi,

This week I finished patching Dan's previous work on the 
`_d_arrayctor` and `_d_arraysetctor` hooks. There was no need for 
a PR to druntime, as the existing code was working fine. The PR 
to DMD is [here](https://github.com/dlang/dmd/pull/13116).

I've also started working on the `_d_arrayappendT` and 
`_d_arrayappendcTX` hooks. As usual, my starting point is [this 
PR](https://github.com/dlang/dmd/pull/9982) form Dan. My current 
status is:
- Trying to fix an error at [this 
line](https://github.com/dlang/dmd/blob/2defede4b40c63528ad208b7b6dc0089be6cdcac/test/runnable/test21403.d#L13) whereby the compiler generates 3 separate calls to `cat11ret3` when appending the final 7.
- Some tests fail because of linker errors related to the 
functions I'm working on. There seems to be an issue with the way 
they're mangled.

Thanks,
Teodor