Release 2020.3.0

2020.03.12: Release 2020.3.0

Infrastructure and requirements changes:

• The install and run-tests scripts have been replaced with a configure
  script (with GNU autoconf-compatible options) and corresponding make all,
  make check and make install steps.
  See INSTALL.md for step-by-step instructions.
• The minimum required GNU Make has risen from 3.79 to 3.80 (released in 2002).
• There is no longer a requirement for Python2.7. While upcxx-run still
  requires a Python interpreter, both Python3 and Python2 (>= 2.7.5) are
  acceptable.

New features/enhancements: (see specification and programmer's guide for full details)

• Added support for non-trivial serialization of user-defined types. See the
  new chapter of the programmer's guide
  for an introduction, and the specification for all
  the details.
• Implement upcxx_memberof(_general)(), enabling RMA access to fields of remote objects
• upcxx::promise now behaves as a CopyAssignable handle to a reference-counted hidden object,
  meaning users no longer have to worry about promise lifetime issues.
• atomic_domain<T> is now implemented for all 32- and 64-bit integer types,
  in addition to the fixed-width integer types, (u)int{32,64}_t.
• atomic_domain<T> implementation has been streamlined to reduce overhead,
  most notably for targets with affinity to local_team using CPU-based atomics.
• Improve GASNet polling heuristic to reduce head-of-line blocking
• Installed CMake package file is now more robust, and supports versioning.
• global_ptr<T> now enforces many additional correctness checks in debug mode
• Significantly improve compile latency associated with upcxx compiler wrapper script
• Improve handling of upcxx-run -v verbose options

Notable issues resolved
(see the UPC++ issue tracker for details):

• issue #83: strengthen global_ptr correctness assertions
• issue #142: Clarify semantic restrictions on UPCXX_THREADMODE=seq
• issue #173: upcxx-run for executables in CWD and error messages
• issue #247: unused-local-typedef warning for some compilers
• issue #266: Redundant completion handler type breaks compile
• issue #267: Same completion value can't be received multiple times
• issue #271: Use of -pthread in example/prog-guide/Makefile breaks PGI
• issue #277: Ensure completion_cx::as_promise(p) works even when p is destroyed prior to fulfillment
• issue #280: Off-by-one error in promise constructor leads to breakage with any explicit argument
• issue #282: Improve installed CMake package files
• issue #287: Bogus install warnings on Cray XC regarding CC/CXX
• issue #289: link failure for PGI with -std=c++17
• issue #294: redundant-move warning in completion.hpp from g++-9.2
• issue #304: Bad behavior for misspelled CC or CXX
• issue #323: Incorrect behavior for global_ptr<T>(nullptr).is_local() in multi-node jobs
• issue #333: Multiply defined symbol detail::device_allocator_core<cuda_device>::min_alignment w/ std=c++2a
• spec issue 148: Missing const qualifiers on team and other API objects
• spec issue 155: value argument type to value collectives is changed to a simple by-value T

This library release mostly conforms to the
UPC++ v1.0 Specification, Revision 2020.3.0.
The following features from that specification are not yet implemented:

• view buffer lifetime extension for remote_cx::as_rpc (issue #262)
• boost_serialization adapter class is not yet implemented (issue #330)
• future<T&&>::{result,wait}_reference currently return the wrong type.
  E.g. future<int&&>::result_reference() returns int const& instead of int&& (issue #313)

Breaking changes:

• atomic_domain has been specified as non-DefaultConstructible since UPC++ spec draft 8 (2018.9)
  This release removes the default constructor implementation from atomic_domain (issue #316).
• The trait template classes upcxx::is_definitely_trivially_serializable and
  upcxx::is_definitely_serializable have been renamed such that the "definitely"
  word has been dropped, e.g.: upcxx::is_trivially_serializable.
• future::{result,wait}_moved, deprecated since 2019.9.0, have been removed.
  New future::{result,wait}_reference calls provide related functionality.