suggest to exclude tests and examples

[m4b]

A crates package doesn't need tests or examples, and the manifest format provides https://doc.rust-lang.org/cargo/reference/manifest.html#the-exclude-and-include-fields to help with this.

I suggest to exclude = ["examples", "tests"] which should dramatically reduce size of the crates download (it currently shows up quite large at 2MB, (e.g. Downloaded 4 crates (2.1MiB) in 0.85s (largest was ``flac-codec`` at 2.0MiB)), most likely due to the flac files in tests, for example).