support for anonymous function intercepts

Allow intercept functions passed to rt_intercept:add/2 to be anonymous. In
compiled code they can either be a plain anonymous function, assuming they
don't use any variables from the surrounding context, or they can be a
2-tuple like this:

{[FreeVar1, ...],
 fun(Arg1, ...) -> ... end}

where FreeVar1 etc. is a list of free variables to be closed over so that
they can be used within the anonymous function. For making interactive
calls to rt_intercept:add/2 from the Erlang shell, only the anonymous
function form is required, even if it uses free variables, though the
2-tuple form is also acceptable.

For compiled code, support for anonymous intercept functions is implemented
via a parse transform, and so to use anonymous functions the intercept
structure(s) containing them must be defined directly inline as part of the
final argument to rt_intercept:add/2, i.e., they cannot be first assigned
to a variable that is then used within the argument. This is because the
value of such a variable might not be visible to the parse transform.

Add a description of anonymous function intercepts to the README.