to!string always allocates a new string

[dlangBugzillaToGithub]

schveiguy (@schveiguy) reported this on 2024-09-01T18:55:47Z

Transfered from https://issues.dlang.org/show_bug.cgi?id=24739

Description

This is a bad and obvious performance issue.

```d
struct S
{
   string toString() { return "S"; }
}

void main()
{
   import std.conv;
   assert(S.init.toString.ptr != S.init.to!string.ptr);
}
```

This assert should not pass -- `to!string` shouldn't do anything other than call `toString` on a struct.

In fact, if `S.toString` *does* allocate, `to!string` is suffering from an *extra allocation* -- one for the call to `S.toString`, and one for the allocated string that `to!string` always builds.

The underlying cause is that the default case for `toImpl!(string, T)` is to create an appender and then use format. There should be a case for when `T.toString` is defined.

See https://github.com/dlang/phobos/blob/f7e523bc3c69506c3e7b4da7e78af93ed8547910/std/conv.d#L1107 and https://github.com/dlang/phobos/blob/f7e523bc3c69506c3e7b4da7e78af93ed8547910/std/conv.d#L136