Fixes #168

The first commit cleans up how we handle our various helper modules (similar to what we did for v0.2), but does not make any functional changes.

The second commit enables our std::error::Error implementations for windows, target_os = "fuchsia", target_os = "ios". These are the additional windows and unix targets we supported in v0.1.6. We don't enable the workaround for target_arch = "wasm32" as that would be a breaking change (from newer version) for no_std wasm32 targets.

Here is a history of when we've enabled std::error::Error implementations:

• v0.1.0 - v0.1.6:
  • Initially, we enabled the std feature on most targets

  #[cfg(any(
    feature = "std",
    windows, unix,
    target_os = "redox",
    target_arch = "wasm32",
  ))]
  

• v0.1.7 - v0.1.8:
  • We later restricted the implementations to certain unix targets that needed util_libc

  #[cfg(any(
    feature = "std",
    target_os = "android",
    target_os = "dragonfly",
    target_os = "emscripten",
    target_os = "freebsd",
    target_os = "haiku",
    target_os = "illumos",
    target_os = "linux",
    target_os = "macos",
    target_os = "netbsd",
    target_os = "openbsd",
    target_os = "redox",
    target_os = "solaris",
  ))]
  

• v0.1.9 - v0.1.10:
  • We further restricted things to only the "std" feature

  #[cfg(feature = "std")]
  

• v0.1.11 - v0.1.15
  • We rolled back our v0.1.9 changes (as they were a breaking change)

  #[cfg(any(
    feature = "std",
    target_os = "android",
    target_os = "dragonfly",
    target_os = "emscripten",
    target_os = "freebsd",
    target_os = "haiku",
    target_os = "illumos",
    target_os = "linux",
    target_os = "macos",
    target_os = "netbsd",
    target_os = "openbsd",
    target_os = "redox",
    target_os = "solaris",
  ))]