Implement a better shuffle

[akalin-keybase]

Copy the shuffle code from go 1.10's math/rand.Shuffle, which is unbiased. Reasons for copying:

• Don't have to deal with the awkwardness of trying to make the shuffle function handle errors from crypto/rand.Reader properly. (Or rather, the awkwardness is pushed to the less critical testing code.)
• Works with earlier versions of go.
• Low chance of behavior drift, since rand.Shuffle is covered by compatibility guarantee.
• Only adds 50 or so lines of non-test code.

Also make as much as possible use csprngRead utility function.

Also handle unlikely overflow in receiver count when signcrypting.

Run go vet, golint, and GOARCH=386 tests in CI.

[coveralls]

Coverage increased (+0.2%) to 84.439% when pulling d1ebc1e917a0eba44f64bc4f1ba4ee3d3e6339b3 on akalin/better-shuffle into 0c8b960f9a0d60e5f4e6c43b8169553692aa21b9 on master.

[coveralls]

Coverage increased (+0.2%) to 84.502% when pulling 5cc4032d5bfd98b62201052603111cd58d4b893e on akalin/better-shuffle into 0c8b960f9a0d60e5f4e6c43b8169553692aa21b9 on master.

[akalin-keybase]

Okay, addressed all comments! PTAL