[Grace-core] Collection naming

[Marco Servetto]

> Here is a principle: we should add language extensions only when there is no good alternative.
I still do not understand if sugaring is considered a language
extention or not in your minds, anyway
you can say that
foo.empty().add(a).add(b).add(c)
is equivalent to foo[a,b,c]
and you can use additional parameters as infos for the maps. for example

foo.empty().add(a) key(aa).add(b) key(bb).add(c) key(cc)
can be equivalent to foo[a key(aa), b key(bb),c key(cc)]
or something similar.