Remove most is* functions and make `is` smarter

I changed `is` such that it takes a second parameter which can
be a string (i.e. the result of calling `type(x)`) or an
object, which may be either a constructor function or a
prototype. We check for constructors first and prototypes
second, because I reckon the constructor is more specific than
a prototype way down the chain.

I'd like to get rid of `isTh` as well, but I'm not sure how to
do that without getting really hacky or introducing some sort
of traits on functions returned from higher order functions.
Forgot to remove `isUndef`
Removed `isTh` since `trampoline` is the only user

Will figure out later on if higher order functions should
somehow "tag" the returned functions, such that they can be
inspected more accurately. For now let's go basic.
Swap argument order on `is` and `isnt`

Puts it inline with Clojure's `instance?` and also makes it
simpler to partially apply (from the left, anyway.)
Added `not` for logical testing (see [comment][1])

This will return `true` for values of `false`, `null`, and
`undefined`. Anything else will return `false`.
This obviously goes against JS truthy/falsy behavior, but I
reckon if that's what you'd like just use `!!` or `!`
respectively; or implement `truthy`/`falsy`.

[1]: https://github.com/mstade/funkis/pull/7#issuecomment-40466894
Changed some `is` logic to make it a bit more consistent

`is(null, x)` will return true for `undefined` and `null`,
whereas before it was strict. Reason is if you care about
`null` you probably don't care to distinguish between it and
`undefined`. If you do, explicitly check for `undefined` which
is strict.

Also changed `is` to make use of `not` for checking if
something is logically true. This means anything is said to
*be* if it is logically true. That is, `is` will return true
for anything that isn't `false`, `null`, or `undefined`;
unless it was called with a type parameter in which case it's
more strict.
Merge pull request #7 from mstade/is-and-isnt

Feel confident in merging this now – thanks @sammyt!