Rewrite merge of clause variable tables (in case, try, etc)

erl_lint:icrt_export/4 has been rewritten to make the code fit to the
scoping rules of Erlang.

 * The function should not depend on calling unused_vars/3 because that
   function does not return variables which begins with an underscore,
   something that only matters when emitting warnings. This could cause
   a compiler crash if such a variable was reused afterwards.

 * The variable tables from each clause are first merged together,
   lists:merge/1 is safe to use because they are orddicts and thus
   already sorted. This list is then traversed parallelly to the old
   variable table, again taking advantage of their sorted order.

 * The function do not emit warnings itself, there is no need to pass
   around the lint state. In the same vein, vtunsafe/3 has been rewritten
   to do more things by itself, given that all of its calls were similar.
   Finally, compiled-out code has been removed.

 * This reverts the code in 9ce148b1059e4da746a11f1d80a653340216c468,
   which fixed the compiler crash and made erl_lint remember unsafe
   variables, but forget about unused variables in the process.