add Is()

Signed-off-by: Egor Kovetskiy <[email protected]>

add Unwrap()

Signed-off-by: Egor Kovetskiy <[email protected]>

fix return signature of Collect

Signed-off-by: Egor Kovetskiy <[email protected]>

add Collect

Signed-off-by: Egor Kovetskiy <[email protected]>

add Flatten with an example

Signed-off-by: Egor Kovetskiy <[email protected]>

update go modules