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feat(integer): full constexpr arithmetic, comparison, shifts, conversions (#757)

* feat(integer): full constexpr arithmetic, comparison, shifts, conversions

Resolves #720. integer joins posit (#718), cfloat (#719), bfloat16 (#725)
as a fully constexpr arithmetic type. After this PR:

  using I32 = integer<32, std::uint32_t, IntegerNumberType::IntegerNumber>;
  constexpr I32 a(42), b(7);
  constexpr auto sum  = a + b;     // works
  constexpr auto prod = a * b;     // works
  constexpr auto quot = a / b;     // works (idiv free function)
  constexpr auto rem  = a % b;     // works
  static_assert(b < a);            // works
  using I128 = integer<128, std::uint32_t, ...>;
  constexpr I128 m_prod = I128(1000000) * I128(1000);  // multi-limb works

Promoted to constexpr (or BIT_CAST_CONSTEXPR via cascading):

  Class members:
    integer::operator+= (uint64-limb path uses is_constant_evaluated()
                          dispatch around addcarry intrinsic; multi-limb
                          path uses index loops instead of pointer-loops)
    integer::operator-= (std::cerr in WholeNumber/NaturalNumber TBD branches
                          guarded with !std::is_constant_evaluated())
    integer::operator*= (uint64-limb path: same is_constant_evaluated dispatch
                          around mul128/addcarry; signed and whole/natural
                          branches both promoted)
    integer::operator*=(BlockType) (same dispatch)
    integer::operator/= operator%= (std::cerr guarded)
    integer::operator<<= operator>>= operator&= operator|= operator^=
    integer::logicShiftRight
    cross-template integer<n>(integer<srcbits>) copy ctor (enables
                          multi-limb mul which constructs integer<nbits+1>
                          internally)

  Free helpers:
    idiv_t default ctor
    idiv() free function (std::cerr guarded)
    All free comparison operators (==, !=, <, >, <=, >=) for
      integer-integer and integer-IntType, both... (continued)

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