Add unwrap operator

[ShadelessFox]

An unwrap (?) operator should return early from the function if supplied expression is resulted in none. This could help avoiding writing if x != none everywhere.

Example:

def read_contents() {
    let file = File.open('test.bin')?; # may return early if File.open resulted in `none`
    let data = file.read()?; # may return early if File.read resulted in `none`
    return data;
}

Alternate syntax: The try keyword can be used for unwrapping as an unary operator.