Save return value using threads

[StLa]

@Zoom Thanks. I now am using "parallel". Now it works. The return values of a 
function in a thread can now be saved. Just an example how to use it:
    
    
    {.experimental: "parallel".}
    
    import httpclient, threadpool
    
    proc fetch(url: string): string =
        return getContent(url)
    
    let urls = ["https://example.com";, "https://example.org";, 
"https://example.net";]
    var res = newSeq[string]()
    
    var tasks = newSeq[FlowVar[string]](urls.len)
    
    parallel:
        for i in 0 ..< urls.len:
            tasks[i] = spawn fetch(urls[i])
    
    for task in tasks:
        res.add(^task)
    
    echo res
    
    
    Run