Fix #[dotenvy::load] and #[async-std::main] used on one function

[mantmorre]

Will eventually close #117

Adds support for async-std along with the new load macro for dotenvy. Also includes an example with tokio and the new macro.

(Apologies for creating a second PR. I did not properly name my branch before opening the previous one.)

[mantmorre]

@allan2 While the process might be convoluted, is there anything inherently wrong with the changes I made?
If dotenvy is the first thing to be run, and it doesn't use any asynchronous calls, what is the requirement to have it be in a synchronous function?

[allan2]

Please read the std::env::set_var safety notice.

This makes it not practically possible for you to guarantee that no other thread will read the environment, so the only safe option is to not use set_var or remove_var in multi-threaded programs at all.

Two comments:

• #[load] should be on top of #[async_std::main]
• no need to have the expansion output in the repo
• just an example parallel to the tokio example will suffice