linked_list implements a generic linked list.  It allows duplicate keys (but only inasmuch as it does not check for them).  We expect to be able to insert arbitrary key-and-value pairs into the a list and then find them later by searching for the key.

We perform two test cases:
test #1: keys of type double, values of type double
test #2: keys of type string, values of type double

Throughout this file, we use the "key => value" notation in order to succinctly denote a key-and-value pair.

test #1: doubles => doubles

operation          expected result
-----------------  -----------------
insert(1. => 2.)
insert(3. => 4.)
insert(5. => 6.)
search(1.)         search returns 2.
search(3.)         search returns 4.
search(5.)         search returns 6.
insert(3. => 7.)
search(8.)         (key not found)
search(3.)         search returns 7.

actual results:

searching for key 1.000000: found value 2.000000
searching for key 3.000000: found value 4.000000
searching for key 5.000000: found value 6.000000
searching for key 8.000000: (not found)
searching for key 3.000000: found value 7.000000

test #2: strings => doubles

operation           expected result
-----------------   -----------------
insert("one", 2)
insert("two", 4)
insert("three", 6)
search("one")       2.
search("three")     6.
search("five")      (key not found)
insert("three", 7)
search("eight")     (key not found)
search("three")     7.

acutal results:

searching for key "one": found value 2.000000
searching for key "three": found value 6.000000
searching for key "five": (not found)
searching for key "eight": (not found)
searching for key "three": found value 7.000000