Auto merge of #275 - bluss:repr-c, r=cuviper
Auto merge of #275 - bluss:repr-c, r=cuviper

Complex: Use repr(C) and add documentation for what it means

Here's an ambitious proposal that puts currently practiced ffi usage of `Complex<f32/f64>` on sound footing.

Fixes  #79

Current users appear to be:

- https://crates.io/crates/blas
  + Their use is not about Complex<f64> in an extern function's signature, but it is explicitly using that it is memory layout compatible.

rust-num/num,rust-lang/num,rust-num/num,rust-num/num-integer,rust-num/num-traits,rust-num/num-integer,koverstreet/num,koverstreet/num,rust-lang/num,rust-num/num-traits,rust-num/num-bigint,rust-num/num-bigint