Don't apply #[no_mangle] pointlessly

[syvb]

It doesn't make sense to apply #[no_mangle] to structs/enums/type aliases, since the names of those items are not mangled, or even directly included in compiled binaries. This was only accepted by rustc accidentally, and newer versions of the compilers warn about this:

warning: attribute should be applied to a function or static
 --> src/ffi/mod.rs:3:1
  |
3 | #[no_mangle]
  | ^^^^^^^^^^^^
4 | #[cfg(feature="std")]
5 | use std::{thread,panic, io, boxed, any, string};
  | ------------------------------------------------ not a function or static
  |
  = warning: this was previously accepted by the compiler but is being phased out; it will become a hard error in a future release!

Furthermore, this can even cause compiler bugs in the latest nightly.

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