OptArgs: Optional arguments for Rust functions

Enable optional arguments for any function:

```rust

[optargs::optfn]

pub fn plot( x: Vec, y: Option>, title: Option<&str>, xlabel: Option<&str>, ylabel: Option<&str>, legend: Option ) {}

// can now call it with optional arguments plot!( x: vec![1,2,3], y: vec![1,2,3], title: "Awesome plot", xlabel: "x axis", ylabel: "y axis" ); ```

...or struct:

```rust

[derive(optargs::OptStruct)]

pub struct Scatter { x: Vec, y: Option>, title: Option<&str>, xlabel: Option<&str>, ylabel: Option<&str>, legend: Option } impl Scatter { fn plot(self) {/* custom impl that references self */} }

// Call it Scatter!{ x: vec![1,2,3], y: vec![1,2,3] }.plot() ```

This crate is especially useful for cleaning up builder-heavy codebases and making library APIs more ergonomic. It also integrates well with Rust-Analyzer and doesn't generate heavy compile times.

This crate adds two macros to make it easy to add optional arguments to functions. - #[optargs] - derive a macro_rules to call a function with optional arguments. - #[derive(OptStruct)] - derive a typed-builder builder for a struct with optional fields.

This crate takes advantage of the recent const_generics in Rust stable (1.51), so our MSRV is 1.51.

Of note: - All optional arguments will default to none. We currently don't provide a custom default (accepting PRs if anyone wants!). - All optional arguments must come after required arguments. - Unnamed positional arguments must be in the correct position. - All arguments can be required, but now you get to name them. - Traditional macrorules scoping applies IE you can't use the macro before function declaration. However, they are exported with macroexport, so you can use them anywhere with crate::$MACRO. Currently, there's no way to disable this, so you can't have two functions with the same name. If this becomes a problem, we'll gladly accept a PR.

How it works:

OptArgs uses const generics to ensure compile-time correctness. I've taken the liberty of expanding and humanizing the macros in the reference examples.