Apply cargo-c on av-metrics

[Luni-4]

Allow C/C++ projects to use av-metrics

• [x] create a capi.rs
• [x] conditionally add it depending on the cargo_c feature
• [x] write an example in the c-example/ directory

[Luni-4]

Missing frame metrics API, here's a prototype

const *MetricType calculate_frame_metric(
    const uint8_t *frame1_data[3],
    const usize frame1_strides[3],
    const uint8_t *frame2_data[3],
    const usize frame2_strides[3],
    uint32_t width,
    uint32_t height,
    uint32_t bitdepth,
    enum Colorspace color
)

[Luni-4]

@dwbuiten

For now I'm no more interested in C-API, so if you want, you can close this issue :)