Impl_drv typo fixes - norton/lets#8 and norton/lets#9

Joe,

Sorry for not coming back to you sooner. This pull request fixes the typos found in the following GitHub issues.

- norton/lets#8: `lets_impl_drv:impl_destroy/6` is calling a wrong port function
- norton/lets#9: A typo in `lets_output_delete2()` (lets_impl_drv.cc)

It also includes a fix in README.md:

- `make eqc` should read `compile-for-eqc`

I tested LETS with eqc 1.26.6, 5000 tests x 2 sets and found no counterexample.

```erl
1> qc_statem_lets:qc_run(5000).
Starting Quviq QuickCheck version 1.26.2
   (compiled at {{2012,6,20},{8,22,48}})
Licence for ****
...
OK, passed 5000 tests

100.00% {1,attempts}

3.740% {{select31,3},ok}
3.732% {{member,2},ok}
3.718% {{select,2},ok}
3.707% {{tab2list,1},ok}
3.702% {{foldl,3},ok}
3.699% {{foldr,3},ok}
3.692% {{select_count,2},ok}
3.691% {{delete,1},ok}
3.676% {{match_delete,2},ok}
3.665% {{select_reverse31,3},ok}
3.661% {{select_delete,2},ok}
3.655% {{select_reverse,2},ok}
3.655% {{match_object31,3},ok}
3.655% {{lookup,2},ok}
3.655% {{insert,2},ok}
3.651% {{all,1},ok}
3.646% {{match,2},ok}
3.640% {{last,1},ok}
3.637% {{delete,2},ok}
3.635% {{match_object,2},ok}
3.633% {{first,1},ok}
3.609% {{match31,3},ok}
3.575% {{new,3},ok}
3.468% {{new,2},ok}
3.189% {{prev,2},ok}
3.100% {{next,2},ok}
2.948% {{lookup_element,3},{error,badarg}}
1.298% {{insert_new,2},ok}
0.671% {{lookup_element,3},ok}
0.513% {{next,2},{error,badarg}}
0.482% {{prev,2},{error,badarg}}
true
(qc_slave@arch-mini)2> qc_statem_lets:qc_run(5000).
...
OK, passed 5000 tests

100.00% {1,attempts}

3.780% {{select,2},ok}
3.763% {{select_reverse,2},ok}
3.753% {{match_object31,3},ok}
3.738% {{match_object,2},ok}
3.725% {{match_delete,2},ok}
3.709% {{foldr,3},ok}
3.678% {{match,2},ok}
3.676% {{select_delete,2},ok}
3.672% {{tab2list,1},ok}
3.659% {{first,1},ok}
3.657% {{select_count,2},ok}
3.656% {{foldl,3},ok}
3.650% {{member,2},ok}
3.650% {{lookup,2},ok}
3.649% {{insert,2},ok}
3.648% {{delete,1},ok}
3.627% {{all,1},ok}
3.625% {{select_reverse31,3},ok}
3.617% {{last,1},ok}
3.611% {{delete,2},ok}
3.602% {{select31,3},ok}
3.591% {{match31,3},ok}
3.533% {{new,3},ok}
3.464% {{new,2},ok}
3.302% {{next,2},ok}
3.230% {{prev,2},ok}
3.088% {{lookup_element,3},{error,badarg}}
1.151% {{insert_new,2},ok}
0.662% {{lookup_element,3},ok}
0.425% {{prev,2},{error,badarg}}
0.410% {{next,2},{error,badarg}}
true
```