custom :get and :set methods not compiled

[Dave Love]

[I noticed this some time ago but hadn't got round to reporting it.
Someone else has noticed in the mean time, but handa thinks no-one is
looking at it, so I'm reporting it here in the hope it doesn't get
forgotten.]

I was surprised to find that lambda forms used for defcustom setters
and getters don't get byte-compiled, which is can be significant in
some cases.  I can't immediately see why they're not compiled.  If
this isn't a bug, it could do with explanation in the doc.