[SAoC 2021] Replace druntime Hooks with Templates: Milestone 4, Week 2

[Teodor Dutu]

Hi,

This week I implemented a template version of `_d_newThrowable` 
and I'm currently working on moving the lowering to it from 
e2ir.d to expressionsem.d. This requires a call to 
`_d_newThrowable`, as well as setting `vthis` and `vthis2`, if 
the exception's class is nested, and then calling its ctor. I'm 
still working on the latter steps of the lowering.

At the same time, I started looking at `_d_arraycatnTX`, starting 
from Dan Printzell's work:
- https://github.com/dlang/dmd/pull/10064
- https://github.com/dlang/druntime/pull/2648/

However, Dan's templated hook is actually just a wrapper that 
calls the old hook. This makes both the hook itself and the 
lowering difficult to maintain and inefficient, because of the 
2D-array that's created by the compiler before being linearised. 
Thus, after I've decided to reimplement `_d_arraycatnTX` to use 
variadic templates instead of the 2D-array trick. This will make 
the implementation much cleaner in both the compiler and in 
DRruntime. I'm currently stuck on the `_d_arraycatnTXTrace` 
alias, however. As it's [currently 
implemented](https://github.com/dlang/druntime/blob/759e60231a12482a1e1df5f891964e270dae0a1b/src/core/internal/array/concatenation.d#L55), the hook for `traceGC` doesn't seem to be instantiable with a variadic template `Tarr`. I'm still exploring solutions for this problem, but I might have to implement the hook myself, without relying on `HookTraceImpl`.

Happy New Year!

Thanks,\
Teodor