Add refutation support

[JonasAlaif]

Adds a refute expr statement, which is an assert expr with errors inverted and without assuming anything in the following statements (an assert expr assumes expr afterwards). That is, a refute expr does not report an error iff an assert expr would. Note that this is not the same as assert !expr since both asserts fail in the following snippet:

var b: Bool
if (*) { assert b }
else { assert !b }

One useful application is to check reachability (e.g. for identifying dead code) since all assertions pass in unreachable code (i.e. refute false will only error in dead code).

To workaround Silicon's numberOfErrorsToReport limit, this PR also adds an ExpectFail info tag to be used on assertions which should not be counted towards the limit (see linked Silicon PR).

• Decide if ExpectFail should be an abstract class instead
• Decide if ⭐ is a good name for a non-deterministic local variable, avoiding clashes
• Add more tests

[mschwerhoff]

Looks good, mostly a few minor comments.

[mschwerhoff]

Looks good, modulo a few minor comments