basic no_std support, as long as you avoid Text and Data and List

[alsuren]

After this patch, the capnp_conv macro works fine in a no_std environment.

I tested by adding #![no_std] to example/src/main.rs and then switching out all Text/Data/List fields in example/example.capnp with Void, and updating main.rs to use () and the println!() with assert!().

If you would like me to add a no_std example folder, I can do. I haven't thought about how to handle Text/Data/List fields in on_std, because my current project doesn't need them. If you want to wait until we have a solution for those types before advertising no_std support, I guess we can just merge this patch as-is.