add a concept for contextual conversion to bool

One concept that is often useful in many types is a contextual conversion to bool. The following expressions should be valid with the concept:

```
if (o) {}
if (!o) {}
template
bool test(O&& o) { return o; }
```

Add a predefined concept that catches these requirement. A possible implementation:

```
namespace boost { namespace type_erasure {

template
struct testable
{
static bool apply(const T& arg)
{ return bool(arg); }
};

template
struct concept_interface, Base, T> : Base
{
explicit operator bool () const
{ return call(testable(), *this); }
};
}}
```

Start by reading the type_erasure concept_interface and call machinery referenced in the issue, then trace how predefined concepts are declared and exposed. Define the contextual bool concept so the listed if, negation, and bool-returning expressions are valid, and verify each expression with the project's existing tests or test conventions.

Written by the indexing model from the issue text.