add a few typechecker hooks

[fredokun]

It would be useful to have some hooks so that the typechecker can be inspected in examples.
As a first guess, the typechecker hooks would be functions of the following form:

__typecheck_HOOK(args...)

As a first example, a simple printer for inferred types would be very useful:

__typlecheck_infer(<expr>)

would print in the output the inferred type of <expr>

Also a simple log printer:

__typecheck_log(args...)

simply prints the args (str(arg1) + str(arg2) + ...)