This change
- Removes interface `log.Format`,
- Removes method `log.FormatFunc`,
- unexports `TerminalHandler.TerminalFormat` formatting methods (renamed to `TerminalHandler.format`)
- removes the notion of `log.Lazy` values
The lazy handler was useful in the old log package, since it
could defer the evaluation of costly attributes until later in the
log pipeline: thus, if the logging was done at 'Trace', we could
skip evaluation if logging only was set to 'Info'.
With the move to slog, this way of deferring evaluation is no longer
needed, since slog introduced 'Enabled': the caller can thus do
the evaluate-or-not decision at the callsite, which is much more
straight-forward than dealing with lazy reflect-based evaluation.
Also, lazy evaluation would not work with 'native' slog, as in, these
two statements would be evaluated differently:
```golang
log.Info("foo", "my lazy", lazyObj)
slog.Info("foo", "my lazy", lazyObj)
```
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.
