The binary-search tests force the practitioner to generify the class
but only one type (integer) was actually being used in the tests.
This should be changed so that more than one type is actually used, to
make it more clear that generics are necessary and to demonstrate how
generics is used.

As discussed in #230, move linked-list to one of the last difficulty 6
exercises, so that binary-seach is the first exercise that introduces
generics.

Add a hint to the binary-search exercise to explain generics as this
is now the first exercise to require it.

Fixes #230.