v2.1.1 Boost 1.71 release

v2.1.1 19th August 2019 (Boost 1.71) [release]

Enhancements:

#184
: As per request from Boost release managers, relocated version.hpp and
revision.hpp into detail, and added the Boost licence boilerplate to the top
of every source file which was missing one (I think). Also took the opportunity
to run the licence restamping script over all Outcome, so copyright dates are now
up to date.

#185
: Add FAQ item explaining issue #185, and why we will do nothing to
fix it right now.

#189
: Refactored the OUTCOME_TRY implementation to use more clarified
customisation points capable of accepting very foreign inputs. Removed the
std::experimental::expected<T, E> specialisations, as those are no longer
necessary. Fixed the documentation for the customisation points which
previously claimed that they are ADL discovered, which they are not. Added
a recipe describing how to add in support for foreign input types.

#183
: Added a separate motivation/plug_error_code specifically for Boost.

Bug fixes:

: OUTCOME_VERSION_MINOR hadn't been updated to 1.

#181
: Fix issue #181 where Outcome didn't actually implement the strong swap guarantee,
despite being documented as doing so.

#190
: Fix issue #190 in Boost edition where unit test suite was not runnable from
the Boost release distro.

#182
: Fix issue #182 where trait::is_exception_ptr_available<T> was always true,
thus causing much weirdness, like not printing diagnostics and trying to feed
everything to make_exception_ptr().

#194
: Fix issue #192 where the std::basic_outcome_failure_exception_from_error()
was being defined twice for translation units which combine standalone and
Boost Outcome's.