Custom pretty printing in utests

[br4sco]

This PR adds the option to supply a function that specifies the output string for the left-hand and right-hand sides of failing utests. The semantics and syntax are best described with an example. The following code snippet:

...
mexpr
let eq : Int -> String -> Bool = lam l. lam r. eqi 0 (string2int r) in
let toString : Int -> String -> String = lam l. lam r.
  join ["left hand: ", int2string l, ", right hand: \"", r, "\""]
in
utest 0 with "1" using eq in               -- Failing
utest 0 with "1" using eq else toString in -- Failing
utest 0 with "0" using eq else toString in -- Passing
()

will on execution output:

 ** Unit test FAILED: <fileinfo> **
    LHS: 0
    RHS: ['1']
    Using: eq

 ** Unit test FAILED: <fileinfo> **
left hand: 0, right hand: "1"
    Using: eq
.ERROR! 1 successful tests and 2 failed tests.

The else function has the signature a -> b -> String and is only applied to the left-hand side and the right-hand side if a test fails (the standard formatting is omitted in this case). Currently, you always have to specify the custom equality function if you want to also specify the else function due to how the bootparser is implemented. We can relax this later on if we want to.
This PR addresses #677.

You can find the above example in test/examples/utest/utest-with-onfail.mc.

[johnwikman]

Looks good! But I would also like to have the option to get LHS: ... RHS: ... formatting for free, i.e. that I just provide string conversion on either side:

let toString : Int -> String -> (String, String) =
    lam l. lam r. (int2string l, concat "strint: " r)
in
utest 0 with "1" using eq onfail toString in
-- This would output
-- ** Unit test FAILED: <fileinfo> **
--    LHS: 0
--    RHS: strint: 1
--    Using: eq

-- or even more convenient
utest 0 with "1" using eq onfail (int2string, concat "strint: ") in

[br4sco]

I propose that we implement that functionality as a library function defaultOnfail : all a. all b. (a -> String) -> (b -> String) -> a -> b -> String rather than adding more keywords and variations on the utest term. Or do you want to replace the current signature so that "RHS:" and "LHS:" are always printed?

[johnwikman]

Yes that sounds like a good solution. Can probably be part of a separate PR then

[br4sco]

Changed onfail to else.