[SAoC 2022] Replace druntime Hooks with Templates: Milestone 1, Week 1

[Teodor Dutu]

Hi,

My project wants to replace dmd hooks to druntime functions such 
as `_d_arraycatnTX`, `_d_newclass` and `_d_newitemT` with 
templates. This will make lowerings to these functions faster at 
runtime, as their templated versions will not require all the 
`TypeInfo`-related indirection used by the current hooks. As a 
result, the maintainability of the compiler will be improved as 
lowerings will now take place in the semantic analysis phase, as 
opposed to the IR phase (where the hooks are currently present). 
In addition, the compiler will also infer function attributes 
such as `pure` and `@nogc` when lowering to the new hooks. The 
compiler will only instantiate those DRuntime functions that the 
app actually calls, thus reducing the size of D executables. This 
will make deploying such apps on microcontrollers easier.

This is a continuation of my project from last year, during which 
I converted 10 hooks to templates:
```
_d_arrayctor
_d_arraysetctor
_d_arrayassign
_d_arrayassign_l
_d_arrayassign_r
_d_arraysetassign
_d_delstruct
_d_newThrowable
_d_arrayappendT
_d_arrayappendcTX
```

The project's goals are stated in [this 
issue](https://github.com/dlang/projects/issues/25) and it is 
tracked in [this 
project](https://github.com/orgs/dlang/projects/10/views/1).

I am currently working on translating `_d_arraycatT` and 
`_d_arraycatnTX` to templates. Currently `_d_arraycatT` was used 
for `a ~ b`, whereas `_d_arraycatnTX` is used for chained 
concatenations such as `a ~ b ~ c`. The latter lowering allocates 
a temporary array on the stack that stores `a`, `b` and `c`: `[a, 
b, c]` then passes this array to the hook as an argument. This is 
inefficient and makes for an inelegant lowering. I intend to 
replace both `_d_arraycatT` and `_d_arraycatnTX` with a new 
`_d_arraycatnTX` that takes arguments as a variadic template. 
Thus, it should cover all use cases without allocating 
unnecessary arrays.

So far, I have implemented and tested the new `_d_arraycatnTX` 
hook. I have also come up with a draft lowering that manages to 
compile druntime and phobos. This was not easy, as they make 
extensive use of array concatenations, especially for strings. 
Now I am working on fixing the lowering and the hook to make the 
tests in dmd pass.

Thanks,\
Teodor