Review: A new stab at a potential std.unittests

[Lutger Blijdestijn]

I'm not particularly fond of this interface and think that a solution with a 
delegate / lazy or alias template parameter would be more convenient. 
However, until we have ast macros I do see the added value in this approach.

Some remarks about the api, not a proper review of the code itself:

- core.exception and std.string must be imported to use the module, relevant 
symbols should be selectively and publicy imported instead.

- exception would be better abbreviated as ex instead of exc, I believe this 
is more conventional. (assertExThrown instead of assertExcThrown)

- assertExcThrown should fail if an exception is thrown that is not T or a 
subclass of T. (catch and throw AssertError)

- assertEqual and assertNotEqual is inconsistent in naming with the 
assertOpFoo class of functions

I believe these assertions should be added:

- assertExcThrown and assertExcNotThrown with a default T (should be 
Exception, not Throwable)

- something equivalent to std.algorithm.equal, the latter is very useful in 
unittests

- assertOpBinary, assertTrue and perhaps assertPred (where the predicate is 
an template alias parameter)