Add IndexMap::into_{keys,values}

[taiki-e]

This adds equivalents of the same methods of HashMap and BTreeMap stabilized in Rust 1.54.

• https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.into_keys
• https://doc.rust-lang.org/std/collections/struct.HashMap.html#method.into_values
• https://doc.rust-lang.org/std/collections/struct.BTreeMap.html#method.into_keys
• https://doc.rust-lang.org/std/collections/struct.BTreeMap.html#method.into_values

[cuviper]

Looks good, thanks!