Allow underscores at the start of a variable

[jbdutton]

Allow one or more underscores at the start of a variable name, like so:

fn main() {
    let _a = 0;
    _a = 42;
    _a
}

This changes the lexer to allow underscores at the start of an identifier, and only emits ask::Kind::Underscore if it's strictly _.

Interestingly, this seems to match the behavior in Rust. They both treat more than one underscore as a valid identifier. So this would work in both Rust and Rune:

fn main() {
    let ___ = 0;
    ___ = 42;
    ___
}

(Assuming you mark ___ as mut in Rust.)

Personally, I find this behavior odd. But maybe matching the Rust compiler behavior is best?

Fixes #435.

[udoprog]

Short and sweet. Thank you!

The msrv error is not your fault, it's an upstream dependency I'll have to deal with once I bake a new release.