attempt to fix/lessen some problems mentioned in issue #9

[corbym]

Splits up the Length, Empty and Nil matchers.

Unfortunately, for matchers that work on arrays, maps or pointers will still require the compiler to be told the type of the underlying value.
E.g.

// nil pointer, actual is declared as `*string`
then.AssertThat(testing, values.actual, is.NilPtr[string]())
//[]int actual 
then.AssertThat(stubTestingT, test.actual, has.Length[int](test.expected))
// map
then.AssertThat(t, map[string]bool{"hello": true}, has.MapLength[string, bool](1))

.. but vanilla Nil only works on errors and requires no boilerplate types:

then.AssertThat(stubTestingT, someError, is.Nil())

.. would this be more acceptable nitram509?
fixes #9

[coveralls]

Coverage: 100.0%. Remained the same when pulling 9e32883 on fix-issue-#9 into 2004755 on master.