redo headless.lang tests to use fixture.FunSuite

instead of gross mutable stuff

the added "fixture =>" and "implicit fixture =>" stuff is kind of
noisy, but better than resorting to mutability IMO.

off in headless.model, Josh Cough chose to stash the fixture
in a DynamicVariable to avoid the extra noise. it's something
we could consider.

also rename stuff in headless.lang (sorry part of same commit; I got
all tangled up in git)