[copybara-service]

Tighten absl::BitGenRef binding rules and simplify its implementation.

• Reject binding to const generators. Generating a variate mutates the
  generator, but BitGenRef previously accepted const URBG& and later
  invoked the non-const operator() through a cast, which is undefined
  behavior for generators defined const. Such constructions now resolve to
  a deleted constructor.
• Detect operator BitGenRef() on an lvalue rather than an rvalue. This
  lets types ref-qualify the operator (operator BitGenRef() &) to
  prevent binding a BitGenRef to a temporary; previously such types were
  silently wrapped as a URBG instead, adding a layer of indirection.
• Merge the mocking and non-mocking constructors; the mock dispatch
  function is now selected with if constexpr.
• Store the generator as void* instead of uintptr_t, replacing
  reinterpret_casts with static_casts, and use a null mock function
  pointer instead of a NotAMock sentinel.