Add unsafe Option::unwrap_unchecked()

[dschatzberg]

This adds a new function to the Option enum which allows for an
unchecked unwrap. In some cases, a developer may be able to statically
determine that an Option must be Some(V) but the compiler cannot. Being
able to forgo a conditional branch is desirable, even if unsafe.

[rust-highfive]

r? @huonw

(rust_highfive has picked a reviewer for you, use r? to override)

[ghost]

This change, as an addition to the standard library, would warrant an RFC, and perhaps a preceding informal discussion.

Personally, I am unsure about the added utility of this method compared to

match x {
    Some(x) => { ... },
    None => intrinsics::unreachable()
}

[dschatzberg]

The unreachable macro is not equivalent as it still panics. This would still force the compiler to check the condition. I have submitted an RFC here: rust-lang/rfcs#1095.

[alexcrichton]

I'm going to close this in favor of the RFC, but we can reopen if it's accepted!