Support parsing half-open ranges in patterns.

[Arnavion]

Fixes #117

[dtolnay]

Looks good to me but hmm, any idea why libsyntax uses two different enums for Expr ranges vs Pat ranges?

pub enum ExprKind {
    /* ... */

    /// A range (`1..2`, `1..`, `..2`, `1...2`, `1...`, `...2`)
    Range(Option<P<Expr>>, Option<P<Expr>>, RangeLimits),
}

/// Limit types of a range (inclusive or exclusive)
pub enum RangeLimits {
    /// Inclusive at the beginning, exclusive at the end
    HalfOpen,
    /// Inclusive at the beginning and end
    Closed,
}

pub enum PatKind {
    /* ... */

    /// A range pattern, e.g. `1...2` or `1..2`
    Range(P<Expr>, P<Expr>, RangeEnd),
}

pub enum RangeEnd {
    Included,
    Excluded,
}

[Arnavion]

Nothing that's obvious from the commit message or PR discussion. @oli-obk ?

[oli-obk]

I think the answer is rather obvious: I didn't know about RangeLimits... XD