Require comma separators for all expression types except the plain block

convert doc-attributes to doc-comments using ./src/etc/sugarise-doc-comments.py (and manually tweaking) - for issue #2498

Instead, use vec::to_mut/from_mut to transform vectors in place as
needed.

Now that core exports "option" as a synonym for option::t, search-and-
replace option::t with option.

The only place that still refers to option::t are the modules in libcore
that use option, because fixing this requires a new snapshot
(forthcoming).

To match the init_fn() and init_fn_mut() changes.

And remove support for the old syntax

Issue #1390