add string-append builtin

Is this what you wanted for string concatenation?

````
> (string-append "foo")
;Value: "foo"

> (string-append "foo" "bar")
;Value: "foobar"

> (string-append "foo" "bar" "baz")
;Value: "foobarbaz"
````

Let me know, I'd be glad to implement more string functions. Thanks for the great project!