This updates the standard library's documentation to use the new syntax. The
documentation is worthwhile to update as it should be more idiomatic
(particularly for features like this, which are nice for users to get acquainted
with). The general codebase is likely more hassle than benefit to update: it'll
hurt git blame, and generally updates can be done by folks updating the code if
(and when) that makes things more readable with the new format.
A few places in the compiler and library code are updated (mostly just due to
already having been done when this commit was first authored).
As per T-lang consensus, this uses a branch to handle the dangling case.
The discussed optimization of only doing the branch in the T: ?Sized
case is left for a followup patch, as doing so is not trivial
(as it requires specialization for correctness, not just optimization).
