Prevent unconditional joins, and naked bool in where() or boolean expressions

- `.from(t1, t2)` produces an unconditional join if you forget to add a condition in the .where()
  sqlpp11 therefore now deprecates unconditional joins.
- more often than not, writing  something like `where(name == "doe")`, you
  meant to write `where(t.name == "doe")`. It is hard to find bugs when
  the former expression compiles because you happen to have a variable
  `name` in the current scope as well.
  sqlpp11 therefore now deprecates `.where(bool)` and disallows
  raw bool values boolean expression like `something and bool`
  wrap bools in sqlpp::value(), if you REALLY want a bool value here

test_static_asserts/where.cpp

@@ -31,7 +31,6 @@
 namespace
 {
   constexpr auto t = test::TabBar{};
-  constexpr auto f = test::TabFoo{};
 
   template <typename T>
   void print_type_on_error(std::true_type)