Support the Write trait when std feature is enabled

[mdsteele]

Many other hash function crates (like md5 or crypto-hash) have their hash objects support the std::io::Write trait, with any write calls updating the hash with the given data. This can be very handy, allowing things like using std::io::copy() to easily hash the entire contents of an artibrary Read type without having to read all the data into memory at once.

I recently found myself wishing for this capability in the sha1 crate too, so this PR adds a Write impl for the Sha1 type (only when the "std" feature is enabled, of course).