Revisions don't compare as numbers

[cmbrandenburg]

CouchDB revisions comprise a number followed by a hash value. A revision with a smaller number is less than a revision with a bigger number.

However, the Revision type compares the number parts lexicographically. E.g., 7 should be less than 13, but the Revision type orders 13 before 7 because the character '1' is less than the character '7'.

The following program demonstrates the problem by failing the assertion:

let r1 = couchdb::Revision::from("7-12345678123456781234567812345678");
let r2 = couchdb::Revision::from("13-12345678123456781234567812345678");
assert!(r1 < r2);

[cmbrandenburg]

As of v0.3.x, the API exposed by the Revision type is fundamentally flawed. The type thinly wraps a String by implementing traits such as AsRef<&str>, From<String>, etc., but, conceptually, the Revision type should instead wrap a tuple comprising the number part (of u64 type) and the hash part (of a string or binary type). So, for example, the type should not implement the AsRef<&str> or From<String> traits but should instead implement std::str::FromStr.