rustler: add a Rust type ErlOption<T>

[tatsuya6502]

This PR fixes #500 by adding a Rust typeErlOption<T> to the Rustler crate.

ErlOption<T> is a wrapper around Option<T>. It implements Rustler's Encoder and Decoder, and serializes None into undefined atom instead of nil. The undefined atom is commonly used in Erlang libraries to express the absence of a value.

For convenience, ErlOption<T> implements some traits from std, so that it will be easy to get a reference of enclosing Option<T> or convert from/to Option<T>. Here are the examples from its doc:

use rustler::ErlOption;

// Create new `ErlOption<i32>` values via convenient functions.
let _ = ErlOption::some(1); // Wraps `Some(1)`.
let _ = ErlOption::<i32>::none();

// Convert Option<i32> values to ErlOption<i32> values.
let _ = ErlOption::from(Some(2));
let _: ErlOption<_> = Some(3).into();
let _: ErlOption<i32> = None.into();

// Get a reference of enclosing Option<T> from an ErlOption<T>.
let _: &Option<i32> = ErlOption::some(4).as_ref();

// Get a mutable reference of enclosing Option<T> from an ErlOption<T>.
let _: &mut Option<i32> = ErlOption::some(5).as_mut();

// Convert an ErlOption<i32> value to an Option<i32> value.
let _: Option<i32> = ErlOption::some(6).into();

// Compare ErlOption<T> with Option<T>.
assert_eq!(ErlOption::some(7), Some(7));
assert!(ErlOption::some(8) > Some(7));

// Call Option<T>'s methods on an ErlOption<T> via Deref and DerefMut.
assert!(ErlOption::some(9).is_some());
assert_eq!(ErlOption::some(10).unwrap(), 10);
assert_eq!(ErlOption::some(12).map(|v| v + 1), ErlOption::some(13));

[filmor]

Maybe, instead of incorporating many small adjustments to the current error-handling, we could try to come up with a system that allows for all of these in one go, e.g. by extending the nif macro. I don't have a proposal for that, yet. @hansihe Do you remember what you had in mind for #2? :)

[tatsuya6502]

ping @filmor @hansihe

[tatsuya6502]

@filmor @hansihe — Can you please decide and tell me what I should do with this PR? Thanks!

[tatsuya6502]

ping @filmor @hansihe