It is true for any decorator, that it makes things faster if
def+decorator is used globally instead of at runtime, but for @func it
is especially true since @func, using decorator.decorate, has relatively
high overhead if it is used not only once at program startup, but
instead every time something is run.

In particular moving @func out of WorkGroup.go() make things
considerably faster:

	name             old time/op  new time/op  delta
	workgroup_empty   195µs ± 0%   113µs ± 1%  -41.91%  (p=0.008 n=5+5)
	workgroup_raise   221µs ± 1%   137µs ± 1%  -38.29%  (p=0.008 n=5+5)

See

	https://lab.nexedi.com/kirr/misc/raw/009c4fee/pygolang/prof_workgroup_empty.svg

for bench_workgroup_empty profile, where it is seen that
decorator.decorate was using ~ half of the whole WorkGroup.go() time.