Use setter `seq.len = 10` instead of `seq.setLen(10)`

[al6x]

It's more consistent with getting the length echo seq.len and looks better.

It's probably not worth to deprecate setLen, too breaking change, but it may be worth to add len= setter and put a note in docs and manual that setLen is older version.

[Araq]

But then we have two ways to write the same and people would be "confused" about whether there is a difference between s.setLen 0 and s.len = 0. Not worth it.