v2.1.4 Boost 1.74 release

v2.1.4 14th August 2020 (Boost 1.74) [release]

Enhancements:

• BREAKING CHANGE void results and outcomes no longer default construct types during explicit construction
  : Previously if you explicitly constructed a result<T> from a non-errored
  result<void>, it default constructed T. This was found to cause unhelpful
  surprise, so it has been disabled.

• New macro OUTCOME_ENABLE_LEGACY_SUPPORT_FOR
  : The macro `OUTCOME_ENABLE_LEGACY_SUPPORT_FOR`` can be used to
  enable aliasing of older naming and features to newer naming and features when
  using a newer version of Outcome.

• Concepts now have snake case style naming instead of camel case style
  : When Outcome was first implemented, it was thought that C++ 20 concepts were
  going to have camel case style. This was changed before the C++ 20 release, and
  Outcome's concepts have been renamed similarly. This won't break any code in
  Outcome v2.1, as compatibility aliases are provided. However code compiled
  against Outcome v2.2 will need to be upgraded, unless OUTCOME_ENABLE_LEGACY_SUPPORT_FOR
  is set to 210 or lower.

• Concepts now live in OUTCOME_V2_NAMESPACE::concepts namespace
  : Previously concepts lived in the convert namespace, now they live in their
  own namespace.

• New concepts basic_result<T> and basic_outcome<T> added
  : End users were finding an unhelpful gap in between is_basic_result<T>
  and value_or_error<T> where they wanted a concept that matched
  types which were basic_result, but not exactly one of those. Concepts filling
  that gap were added.

• Operation TRY works differently from Outcome v2.2 onwards
  : This is a severely code breaking change which change the syntax of how one uses
  OUTCOME_TRY(). A regular expression suitable for upgrading code can be found in
  the list of changes between Outcome v2.1 and v2.2.

Bug fixes:

#224
: The clang Apple ships in Xcode 11.4 (currently the latest) has not been patched
with the fixes to LLVM clang that fix noexcept(std::is_constructible<T, void>)
failing to compile which I originally submitted years ago. So give up waiting on
Apple to fix their clang, add a workaround to Outcome.

Spare storage could not be used from within no-value policy classes
: Due to an obvious brain fart when writing the code at the time, the spare storage
APIs had the wrong prototype which prevented them working from within policy classes.
Sorry.