DConf '22: No-Allocated 0-terminated path strings

[cc]

Catching up on the DConf '22 videos, really enjoyed the tricks 
Walter presents here for no-allocation strings:

[DConf '22: Strawberries and Cream aka Delightful Emergent 
Properties of D -- Walter 
Bright](https://www.youtube.com/watch?v=iuP-AWUyjp8)

In the Q&A segment, the first question asked whether the filename 
in the path example needs memory allocation to be passed to C, 
and is told it does, however the two methods presented can be 
easily combined to provide no-allocation 0-terminated strings 
from `chain()`ed paths by passing an `InputRange` rather than a 
`const(char)` to `toCStringThen`:

```d
//version=AllowMalloc;
auto toCStringThen(alias dg, Range)(Range src) /*nothrow*/ if 
(isInputRange!Range && !isInfinite!Range) {
	const len = src.walkLength;
	char[512] small = void;
	version(AllowMalloc) {
		import dmd.common.string : SmallBuffer;
		auto sb = SmallBuffer!char(len + 1, small[]);
		scope ptr = sb[];
	} else {
		enforce(len < small.length, format!"C string buffer overflow 
(%s >= %s)"(len, small.length));
		scope ptr = small[];
	}
	size_t i = 0;
	foreach (char c; src)
		ptr[i++] = c;
	ptr[len] = '\0';
	return dg(ptr);
}
void main() {
	string path = "include/";
	string name = "file";
	string ext = ".ext";

	auto filename = chain(path, name, ext);
	filename.writeln;
	filename.byChar.toCStringThen!(
		(str) => printf("printf: {%s}\n", str.ptr)
	);
}
```
May need to be cleaned up for character types and needs to 
iterate twice if allocations are going to be allowed, and 
walkLength/chain()'s range functions aren't nothrow as far as I 
can tell.  But otherwise thought this was kind of neat, just 
posting it here in case anyone else finds it handy.