[SAoC 2022] Replace druntime Hooks with Templates: Milestone 1, Week 3

[Teodor Dutu]

Hi,

This week, with the help of my mentors, I fixed the type 
mixmatches with `_d_arraycatnTX`. Then I fixed the bugs in my 
lowering and hook so that now all tests in druntime and phobos 
pass. The only failing test in dmd is [`test19688.d`](). The 
reason it fails is that passing `__FUNCTION__` as an argument to 
another function makes it evaluate to an empty string, like in 
the code below:

```d
string foo(string arg)
{
     return arg;
}

T fooT(T)(T arg)
{
     return arg;
}

void bar(string s = fooT(__FUNCTION__))
{
     assert(s != "", s);  // this fails
}

void baz(string s = foo(__FUNCTION__))
{
     assert(s != "", s);  // this fails
}

void taz(string s = __FUNCTION__)
{
     assert(s != "", s);  // this passes
}

void main()
{
     taz();
     baz();
     bar();
}
```

I am not sure whether this is a bug or not and I am still 
investigating the issue. Once I figure it out and fix it or file 
a bug report, I'll raise a PR with the new `_d_arraycatnTX` hook. 
Then I'll continue with `_d_newitem{T,iT,U}`.

Thanks,
Teodor