First of a small number of new allocator issues

Also, moved two 'constexpr' issues to 'Resolved' status, based on
papers adopted at the recent Portland meeting.
Jonathan's update of #2210
New issue from Jonathan in regard to ambiguous wording for allocators
Minor editorial improvement
Improve P/R a bit
Repurpose issue 2212 before it resolves as a duplicate
Fix example in issue 2212

The whole point of the issue is to add the 'const' keyword,
but the example omits it.  Oops.
Fix formatting in issue 2212

Yikes, the last commit was also too hasty.  Fixed numerous
formatting issues with XML entities and the like, and also
clarified the 'discussion' text a some small but useful
ways.
Fix typos, improve formatting
Resolve another issue, addressed last meeting
Adding Pete's new issue in regard to regex_replace including P/R
New issue from A. Semashev in regard to IO init()
Add issue 2215
New regex_replace issue with allocators from Jeffrey
New issue from Jeffrey in regard to some submatch comparisons
New issue from Jonathan in regard to allocator_traits::construct and containers
Add new issue from Jon in regard to INVOKE with reference_wrapper
fix typo in 2198 title
Attempt to simplify P/R of 2219

The type U introduced by the inserted text can be used to simplify the existing wording.
Merge branch 'patch-2' of https://github.com/jwakely/LWG