Let's bikeshed std.experimental.testing assertions/checks/whatchamacallits

[Atila Neves via Digitalmars-d]

In case you don't know what I'm talking about: 
https://github.com/D-Programming-Language/phobos/pull/3207

Since this is an API issue it's import to get it right the first 
time. Personally I'm not sure what I prefer (well, I am, but what 
I actually want isn't syntactically valid D). I think the options 
so far are:

1) What's there already, namely `shouldEquals`, `shouldBeIn`, etc.
2a) Compile-time strings for operators: `should!"=="`, 
`should!"in"`
2b) Dicebot's `test!"=="`. `assert` is so much better, I wish we 
could use that.
3) Composable ones: should.equals, should.not.equals, or another 
word that isn't "should"
4) Anything else?

I'm not convinced composability brings anything to the table 
except for editor dot-completion. I don't like the verbosity of 
what's there now, but my prefererred syntax doesn't work except 
for the ubiquitous  check for equality (`should ==`). Well, the 
dream would be that `assert(foo == bar)` did what part of this PR 
does, but that's another story and something that can't be done 
by a library unless we had AST macros, which we won't. Or Lisp's 
reader macros, but we won't get those either.

Thoughts? Votes?

Atila