[SAoC 2021] Replace druntime Hooks with Templates: Milestone 1, Week 3

[Teodor Dutu]

Hi,

The changes to DMD from the [previous 
week](https://github.com/dlang/dmd/pull/13116) made some druntime 
unittests fail. Specifically, some of those in `core.lifetime`, 
because the existing `_d_arrayctor` implementation could not be 
`@nogc`.

I spent this week fixing the above issue by removing the call to 
`enforceRawArraysConformable` from `_d_arrayctor`, and performing 
its checks "manually". `enforceRawArraysConformable` prevented 
`_d_arrayctor` from becoming `@nogc` because it created its error 
messages by string concatenation, which is not `@nogc`.

The PR that _should_ fix these issues is 
[here](https://github.com/dlang/druntime/pull/3582).

I haven't been able to work on `_d_arrayappendT` and 
`_d_arrayappendcTX`, but I plan to catch up next week.

Thanks,
Teodor