Extend Encryption API, Improve KeyGen performance

[cfromknecht]

This PR extends the encryption API of this package with:

• EncryptWithNonce - Encrypts a plaintext with user provided nonce.
• EncryptAndNonce - Generates a random nonce, and returns it to the user along with the ciphertext.

It also makes a simple optimization to generate p and q in parallel during keygen, offering a modest speed up.

Primes generate concurrently:

goos: darwin                                                       
goarch: amd64                                                      
pkg: github.com/roasbeef/go-go-gadget-paillier                     
BenchmarkKey1024-8           100          16601491 ns/op          878931 B/op       3873 allocs/op                                     
BenchmarkKey2048-8            10         126341031 ns/op         1913843 B/op       6138 allocs/op                                     
BenchmarkKey3072-8             3         615750569 ns/op         4517426 B/op      11692 allocs/op                                     
BenchmarkKey4096-8             2        2607939290 ns/op        11143432 B/op      23340 allocs/op                                     
PASS                                                               
ok      github.com/roasbeef/go-go-gadget-paillier       12.398s 

Primes generated serially:

goos: darwin
goarch: amd64
pkg: github.com/roasbeef/go-go-gadget-paillier
BenchmarkKey1024-8            50          26715935 ns/op          972709 B/op       4209 allocs/op
BenchmarkKey2048-8            10         168290016 ns/op         1655725 B/op       5373 allocs/op
BenchmarkKey3072-8             3         803726192 ns/op         4932906 B/op      12703 allocs/op
BenchmarkKey4096-8             1        3289647242 ns/op        10710200 B/op      22465 allocs/op
PASS
ok      github.com/roasbeef/go-go-gadget-paillier       9.172s

Finally, the benchmarks have been modified to report allocations and properly reset the timer after initializing the relevant benchmarks. Several calls to Fatalf have also been corrected to use a formatting directive, allowing the package and tests to used with Go1.11. Without the directive, the compilation would fail due to the stricter vet checks at compile time.

[Roasbeef]

LGTM 🦎