Add support for SyncFunction

[udoprog]

Regular Functions are not thread-safe, since they might contain a closure which has captured some of the runtime state of Rune.

This adds support for SyncFunction, which takes ownership of the runtime state that it's captured and converts it into a ConstValue. ConstValues are an owned variation of Value which does not perform the same ownership work as Value.

This currently does have the effect of moving any captured values, which really should be implemented in two separate ways. And the concepts map quite nicely to rust's move operator.

let value = /* .. */;

// This should clone the value if converted to a sync function.
let a = || {
    dbg(value);
}

let value = /* .. */;

// This should move value.
let b = move || {
    dbg(value);
}