add a proc macro version

[csos95]

This fixes #13 and fixes #15.

[csos95]

I just realized that this proc macro doesn't support using a custom cache type.
I'll add more attributes to support this and a proc macro version of the kitchen_sink example

• type = "Type" - set cache type
• create = "Block" - set cache creation block

Like with the key and convert attributes, it's going to have to be strings that get parsed separately.

[jaemk]

This is awesome, thank you!

[csos95]

Okay, I got those added. I will add some documentation to the proc macro and then this should be good to go.

[csos95]

I added the documentation and since adding proc macros can sometimes increase compile time by quite a bit if there aren't already dependencies using them, I added it as a default feature.
If someone wants to use this without the overhead of compiling syn they can add default-features = false to the dependency.

[jaemk]

Nice, thanks! I'll upload a new release later today