Improve compile speed and binary size in opt

[senhuang42]

Remove basically all inlining/templating in opt.
I measure no impact to end-to-end compression speed at all (levels 13-16, silesia.tar).

macOS with gcc

dev:
zstdcli binary size: 1703976
time to compile opt.o: ~8.5s

opt_compiletime:
zstdcli binary size: 1458352  (14.5% reduction)
time to compile opt.o: ~1.8s

The bigger bottleneck is still lazy, which will require a bit more work to reduce without speed regressions.

[Cyan4973]

I measure no impact to end-to-end compression speed at all (levels 13-16, silesia.tar).

I checked that statement.
To begin with, zstd_opt only starts at level 16 for large files, so obviously levels 13-15 will remain identical after this change.

Then, from level 16 to 22, I measured a -5 to -10% speed drop across a few scenarios on my Ubuntu station (using gcc-9.3).

I also measured the mentioned binary size reduction, as well as the sharp compilation time reduction.

So now it's a matter of trade-off, since it's not completely free.