improve performance for some orddict funs

Improve the performance of some orddict funs by reimplementing them using
the lists module in a way that preserves backward compatibility.

The QuickCheck programs linked below were used to verify backward
compatibility:

* https://gist.github.com/vinoski/3bd216efa421c581174a
* https://gist.github.com/vinoski/c6db70e8dc725083843d

Both tests, which were run on R16B02, require the original orddict module
to be renamed to olddict, and that code:unstick_mod/1 be applied to orddict
in order to allow it to be replaced with the revised orddict.

The first QuickCheck test first generates a list of pairs of terms, then
uses the list to create both an original and revised orddict, then randomly
chooses one of the revised orddict functions and applies it to the two
orddict instances, and finally verifies that the results of the operation
are the same for both instances. The second QuickCheck test is similar
except that it first creates an instance of the original and revised
orddicts and then folds over the same randomly-generated list of revised
orddict functions, applying each function to each orddict instance and
verifying that the results match.

The QuickCheck tests originally found one problem in the revised fetch and
find functions related to key comparison between integer and real keys,
which was easily fixed. Many tens of thousands of runs of each test
thereafter revealed no other errors. A number of intentional errors were
also injected into the revised orddict module to make sure the QuickCheck
tests would catch them, and they were caught in all cases. Code coverage
analysis was also applied to verify that the QuickCheck tests were covering
the revised functions; the only revised functions left uncovered were the
trivial clauses dealing with empty orddicts, specifically:

* is_key(Key, [])
* fetch(Key, [])
* find(Key, [])
* store(Key, New, [])

The revised orddict functions were also tested to assess performance
against the original orddict implementation. The test program used is
available here:

* https://gist.github.com/vinoski/61772a052f3501e1e128

Since an orddict instance is implemented as a list, the test program
creates ordicts of length 1, 10, 100, and 1000 and uses them to assess
performance at each length. Performance was measured using timer:tc/3 to
time a number of iterations of various tests against the original orddict
and against the revised orddict. The following tests were applied:

* fetch: Create an orddict of length L, where L is 1, 10, 100, or
  1000. Create a random integer key, which may or may not be present in the
  orddict. For each L, time 10000 attempts to fetch the random key and also
  fetch a key known to not be present in the orddict.

* find: Create an orddict of length L, where L is 1, 10, 100, or
  1000. Create a random integer key, which may or may not be present in the
  orddict. For each L, time 10000 attempts to find the random key and also
  find a key known to not be present in the orddict.

* from_list: Time 1000 attempts to create orddicts of lengths 1, 10, 100,
  and 1000 from a list of random pairs with integer keys. For lengths
  greater than 1, two different tests are performed: one passing a list of
  pairs in sorted key order, and the other passing a list of pairs in
  reverse sorted key order. Since orddicts are ordered, these orderings
  effect worst-case and best-case behavior of the original
  orddict:from_list/2 implementation respectively.

* is_key: In orddicts of lengths 1, 10, 100, and 1000, time 10000 attempts
  to first look for the largest key known to be in the orddict and also
  look for another key known to be larger than any key present.

* store: For lists of random pairs of lengths 1, 10, 100, and 1000, create
  an empty orddict and time 1000 iterations of storing all pairs into the
  orddict.

These tests were performed against R16B02 on a Macbook Pro with an Intel
Core i7 processor running at 2.7GHz and 16GB of RAM running OS X 10.8.5,
and on a Dell system with a 3.4GHz Intel Core i7 and 16GB of RAM running
Ubuntu Linux 12.04.

The tables below show results for OS X and Linux respectively. Each table
lists the name of each test followed by two numbers, each a time in
microseconds of the average of 10 runs of the test. The first number is the
result for the original orddict, the second for the revised orddict.

As the numbers for both platforms show, the revised fetch, find, from_list,
and is_key functions are always faster than the original versions, in some
cases quite a bit faster. Notably, for the from_list cases of length 10,
100, and 1000, the revised version of from_list is 1, 2, and 3 orders of
magnitude faster respectively. For the store function, the revised version
is slightly faster in all cases except for the length 100 case, where the
original store implementation is slightly faster than the revised version.

Results from OS X:
------------------
fetch length 1: 10.14 9.16
fetch length 10: 12.64 10.15
fetch length 100: 39.53 11.60
fetch length 1000: 205.85 18.18
find length 1: 1.31 0.73
find length 10: 4.99 0.88
find length 100: 42.08 2.62
find length 1000: 268.10 13.28
from_list length 1: 1.23 0.31
from_list length 10 ordered: 27.38 9.48
from_list length 10 reverse ordered: 12.24 10.38
from_list length 100 ordered: 1772.07 63.45
from_list length 100 reverse ordered: 87.95 55.87
from_list length 1000 ordered: 171497.21 680.12
from_list length 1000 reverse ordered: 931.64 617.90
is_key length 1: 0.91 0.54
is_key length 10: 3.30 1.36
is_key length 100: 28.34 3.11
is_key length 1000: 331.17 14.38
store length 1: 0.70 0.69
store length 10: 15.18 14.17
store length 100: 881.28 888.42
store length 1000: 62943.43 62227.62

Results from Linux:
-------------------
fetch length 1: 9.23 8.45
fetch length 10: 10.30 8.75
fetch length 100: 18.57 10.72
fetch length 1000: 72.82 15.70
find length 1: 0.40 0.32
find length 10: 1.87 0.54
find length 100: 14.61 2.16
find length 1000: 97.42 11.11
from_list length 1: 0.47 0.12
from_list length 10 ordered: 11.72 3.78
from_list length 10 reverse ordered: 4.87 4.61
from_list length 100 ordered: 718.11 27.69
from_list length 100 reverse ordered: 34.04 27.25
from_list length 1000 ordered: 72121.96 339.10
from_list length 1000 reverse ordered: 466.38 343.22
is_key length 1: 0.27 0.21
is_key length 10: 1.24 0.61
is_key length 100: 10.68 2.09
is_key length 1000: 129.34 12.54
store length 1: 0.36 0.31
store length 10: 5.78 5.43
store length 100: 334.22 346.94
store length 1000: 25687.93 25441.16