Little CMS bindings for Rust

Makes LCMS C API available in Rust. There's also a higher-level wrapper.

Differences from C

* Type names don't have the `cms` prefix, e.g. `cmsColorSpace` is `ColorSpace`.
    * All C function names remained the same (with the prefix, e.g. `fn cmsReadTag()`).
* Enum values don't have the `cmsSig` prefix, e.g. `cmsSigLabData` is `LabData`.
* Some arguments use more specific type, e.g. `Intent::Perceptual` enum instead of `INTENT_PERCEPTUAL` integer.