Add a simple A440 synth example

[ulfmagnetics]

Just taking a crack at solving https://github.com/RustAudio/synth/issues/18. I'm a Rust newbie, but this was pretty straightforward.

I couldn't figure out a way to specify an oscillator frequency without using the synth::dynamic module... if there's a simpler way I'd love to know about it!

[mitchmindtree]

Thanks! 😸

I left a comment inline re- avoiding the synth::dynamic module, hopefully it makes sense and is useful!

BTW, I'll have a PR within the next few days that removes the dynamic module all together in favour of Synth::dynamic and Oscillator::dynamic methods and making better use of rust's impl blocks in general, along with some other slight improvements. I'm happy to merge this though (with those alterations I mentioned in the comment) and then make any necessary adjustments in my PR :+1:

[ulfmagnetics]

Thanks for the very clear explanation! Hopefully this'll get the job done now..