Parse from bytes::Bytes iterator

[shirok1]

Solve my #611 . BytesIteratorReader lacks document and test, please tell me the convention to add them.

[stepancheg]

There's an issue with this PR.

This PR does reading by wrapping Iterator<&Bytes> into Read. Which means CodedInputStream has no option to reuse underlying Bytes objects without copying them into memory. When CodedInputStream is constructed from Bytes, and when Bytes is requested (when tokio_bytes codegen option is used, see CodedInputStream::read_raw_tokio_bytes function), the generated messages bytes reference original Bytes object allocation, not create new allocation (which might be much more efficient).

In current form the PR does nothing which cannot be done outside of the crate.