Constrained relational operators in line with P2944R3

[joaquintides]

Closes #60. Observations:

• The constraints are not 1:1 with those of std::variant because Boost.Variant2 implements operator>/operator>= in terms of operator</operator<=.
• For compilers without expression SFINAE support, constraint checking does not work if any of the corresponding Ti's relops is deleted: in this case, variant's relop fails to instantiate (as before this PR). This limitation is not documented in the reference.

[pdimov]

What C++11 compiler defines BOOST_NO_SFINAE_EXPR?

[joaquintides]

Good question. I got the check from here. According to Claude, the C++11 compilers with BOOST_NO_SFINAE_EXPR are:

• Unconditionally
  • Metrowerks/CodeWarrior
  • GCC-XML
  • PathScale EKOPath
  • MPW MrCpp/SCpp
  • HP aCC
  • Borland C++
  • Digital Mars
  • CodeGear/Embarcadero
  • IBM VisualAge / XL C++ (AIX)
  • IBM XL C++ for z/OS
  • common_edg.hpp
  • Intel on WIndows
• Conditionally
  • GCC < 4.5
  • Visual Studio with _MSC_FULL_VER < 190024210
  • Oracle/Sun Studio before 12.4
  • Cray < 8.5
  • Intel on Linux/macOS <12.0