repr_c_wrapper 0.1.0

Opaque `#[repr(C)]` wrapper that can be passed by value over FFI
Documentation 
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Opaque `#[repr(C)]` wrapper for `#[repr(Rust)]` types that can be passed by value over FFI

**IMPORTANT** Only types requiring 8 Byte alignment or less can be wrapped, and the C
environment must align uint64_t to at least 8 Byte boundaries.

``` Rust
use repr_c_wrapper::*;

#[repr(C)]
pub struct OpaqueWrapper(repr_c_wrapper_t!(String));

#[no_mangle]
pub extern "C" fn some_func() -> OpaqueWrapper {
   OpaqueWrapper("hello".to_string().into())
}
```

**Acknowledgment** Thanks to [@QuineDot](https://github.com/QuineDot), [@H2CO3](https://github.com/H2CO3), and 
[@bruecki](https://users.rust-lang.org/u/bruecki/summary) for identifying unsound practices in earlier drafts of this crate.