LatLng and LatLngBounds should enforce invariants

[jfirebaugh]

Remove operator bool() from both classes and enforce the following invariants.

• LatLng constructor throws if any of the following are violated:
  • std::isfinite(latitude)
  • std::isfinite(longitude)
  • -90 <= latitude <= 90
• LatLngBounds constructor swaps coordinates as necessary to enforce:
  • sw.latitude <= ne.latitude
  • sw.longitude <= ne.longitude

[jfirebaugh]

It's useful to have an "empty" state for LatLngBounds: #3803. We can relax the invariant slightly and keep a bool empty() const method around for that.

[jfirebaugh]

Just discovered an example of how dangerous a non-explicit operator bool can be: currently LatLng(a, b) == LatLng(c, d) is true for any non-NaN a, b, c, d.