These changes improves the performance of the non-coloured terminal formatting, _quite a lot_.
```
name old time/op new time/op delta
TerminalHandler-8 10.2µs ±15% 5.4µs ± 9% -47.02% (p=0.008 n=5+5)
name old alloc/op new alloc/op delta
TerminalHandler-8 2.17kB ± 0% 0.40kB ± 0% -81.46% (p=0.008 n=5+5)
name old allocs/op new allocs/op delta
TerminalHandler-8 33.0 ± 0% 5.0 ± 0% -84.85% (p=0.008 n=5+5)
```
I tried to _somewhat_ organize the commits, but the it might still be a bit chaotic. Some core insights:
- The function `terminalHandler.Handl` uses a mutex, and writes all output immediately to 'upstream'. Thus, it can reuse a scratch-buffer every time.
- This buffer can be propagated internally, making all the internal formatters either write directly to it,
- OR, make use of the `tmp := buf.AvailableBuffer()` in some cases, where a byte buffer "extra capacity" can be temporarily used.
- The `slog` package uses `Attr` by value. It makes sense to minimize operating on them, since iterating / collecting into a new slice, iterating again etc causes copy-on-heap. Better to operate on them only once.
- If we want to do padding, it's better to copy from a constant `space`-buffer than to invoke `bytes.Repeat` every single time.
dd0d0a2522