Do not special-case '_' as a pattern

[klntsky]

This line was really scary to read:

if (patt === '_') {
    // Behaves like UnderscoreType
    return [true, [value]];
}

What if someone would use this library to match a dynamically-constructed string with another one, which can be altered by an adversary? The adversary may be able to inject '_', and then it will be interpreted as a wildcard pattern instead of a literal value, thus resulting in a wrong clause being executed.

For people who have had experience with functional programming, the natural expectation from any pattern matching system is to abide the property:

if b is a concrete value and a is not equal to b, then match(a, b, f) will not lead to f clause execution.

Though of course it is questionable where to draw a line between concrete values and patterns, I insist that special-casing like this is not intuitive and may lead to very bad things.

Moreover, the special-case is not documented at all, which is obviously an issue on its own.