Make rustc-serialize fully portable

[jethrogb]

This PR makes it so that rustc-serialize compiles and has reasonable behavior on all platforms.

Even though rustc-serialize is deprecated, it's a cornerstone of the Rust crates ecosystem. Popular projects such as RLS and rust-crypto depend on it. Not supporting all platforms hampers the growth of new platforms (WebAssembly, SGX, etc.) in favor of existing platforms.

This PR should be a one-time fix for all future platforms that Rust might support in the future.

Fixes #190 Fixes #191 Fixes #194

[kostko]

It would be really nice if this was merged. Anything blocking it? cc @alexcrichton @dtolnay

[jethrogb]

I have published these and other changes as the rustc-serialize2 crate.