chore(*): use 'explicit operator bool' everywhere.

operator bool() may produce unwanted conversions.
For example, we had the following bug in the code base.

...
   object const & obj = find_object(const_name(n));
   if (obj && obj.is_builtin() && obj.get_name() == n)
...

obj.get_name() has type lean::name
n              has type lean::expr

Both have 'operator bool()', then the compiler uses the operator to
convert them to Boolean, and then compare the result.
Of course, this is not our intention.

After this commit, the compiler correctly signs the error.
The correct code is

...
   object const & obj = find_object(const_name(n));
   if (obj && obj.is_builtin() && obj.get_name() == const_name(n))
...

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(*): use 'explicit operator bool' everywhere.