DRuntime: Move Functions from `rt/lifetime.d` to `core.lifetime.d` as Templates

[Teodor Dutu]

Hi,

I am working on converting [`_d_newarray{U,iT,T}` to a 
template](https://github.com/dlang/dmd/pull/15299). 
`_d_newarrayU()` calls `__setArrayAllocLength()`, which is 
[defined in 
`rt/lifetime.d`](https://github.com/dlang/dmd/blob/17c3f994e845ff0b63d7b5f6443fe5a7fdc08609/druntime/src/rt/lifetime.d#L265). Due to how DRuntime is built, `rt/lifetime.d` cannot be imported into `core/lifetime.d` (or `core/internal/lifetime.d`).

To circumvent this, I also moved `__setArrayAllocLength()` along 
with other functions in that call stack such as `__arrayStart()` 
to `core/lifetime.d`. To avoid some linker errors caused by the 
fact that `core/lifetime.d` is not compiled into the 
`libdruntime.a` as it's only meant to contain templates, I 
converted `__setArrayAllocLength()` and the other functions to 
["empty" templates (with no template 
arguments)](https://github.com/dlang/dmd/blob/bde3a7f0d6dd043b7981c8b7cc839607c53ed251/druntime/src/core/lifetime.d#L3130) just to see if they work. They do.

So my approach now is to convert these new templates to "real" 
templates, that use their template argument instead of 
`TypeInfo`. This will help use `TypeInfo` less (and hopefully at 
some point not use it at all in the runtime) but may lead to a 
lot of template bloating. To reduce this, it's worth noting that 
`__setArrayAllocLength()` only uses `TypeInfo` to place it at the 
end of the allocated array. Therefore, the template 
implementation can be split into a small template (that only 
handles this part) and a larger non-template function that does 
everything else (which is common for all types).

Do you agree with this approach or do you have other suggestions?

Thanks,
Teo