Auto merge of #4521 - goffrie:master, r=alexcrichton
authorbors <bors@rust-lang.org>
Sat, 23 Sep 2017 02:50:28 +0000 (02:50 +0000)
committerbors <bors@rust-lang.org>
Sat, 23 Sep 2017 02:50:28 +0000 (02:50 +0000)
Use memoized hashes when hashing Fingerprint.

The recursive hashing of dependencies can cause exponential blowup. We
already have a memoized hash available, so use that, Merkle-tree-style.


Trivial merge