Implementation of Commutative Supersingular Isogeny Diffie Hellman,
based on "A faster way to CSIDH" paper (2018/782).

* For fast isogeny calculation, implementation converts a curve from
  Montgomery to Edwards. All calculations are done on Edwards curve
  and then converted back to Montgomery.
* As multiplication in a field Fp511 is most expensive operation
  the implementation contains multiple multiplications. It has
  most performant, assembly implementation which uses BMI2 and
  ADOX/ADCX instructions for modern CPUs. It also contains
  slower implementation which will run on older CPUs

* Benchmarks (Intel SkyLake):

  BenchmarkGeneratePrivate   	    6459	    172213 ns/op	       0 B/op	       0 allocs/op
  BenchmarkGenerateKeyPair   	      25	  45800356 ns/op	       0 B/op	       0 allocs/op
  BenchmarkValidate          	     297	   3915983 ns/op	       0 B/op	       0 allocs/op
  BenchmarkValidateRandom    	  184683	      6231 ns/op	       0 B/op	       0 allocs/op
  BenchmarkValidateGenerated 	      25	  48481306 ns/op	       0 B/op	       0 allocs/op
  BenchmarkDerive            	      19	  60928763 ns/op	       0 B/op	       0 allocs/op
  BenchmarkDeriveGenerated   	       8	 137342421 ns/op	       0 B/op	       0 allocs/op
  BenchmarkXMul              	    2311	    494267 ns/op	       1 B/op	       0 allocs/op
  BenchmarkXAdd              	 2396754	       501 ns/op	       0 B/op	       0 allocs/op
  BenchmarkXDbl              	 2072690	       571 ns/op	       0 B/op	       0 allocs/op
  BenchmarkIsom              	   78004	     15171 ns/op	       0 B/op	       0 allocs/op
  BenchmarkFp512Sub          	224635152	         5.33 ns/op	       0 B/op	       0 allocs/op
  BenchmarkFp512Mul          	246633255	         4.90 ns/op	       0 B/op	       0 allocs/op
  BenchmarkCSwap             	233228547	         5.10 ns/op	       0 B/op	       0 allocs/op
  BenchmarkAddRdc            	87348240	        12.6 ns/op	       0 B/op	       0 allocs/op
  BenchmarkSubRdc            	95112787	        11.7 ns/op	       0 B/op	       0 allocs/op
  BenchmarkModExpRdc         	   25436	     46878 ns/op	       0 B/op	       0 allocs/op
  BenchmarkMulBmiAsm         	19527573	        60.1 ns/op	       0 B/op	       0 allocs/op
  BenchmarkMulGeneric        	 7117650	       164 ns/op	       0 B/op	       0 allocs/op

* Go code has very similar performance when compared to C
  implementation.
  Results from sidh_torturer (4e2996e12d68364761064341cbe1d1b47efafe23)
  github.com:henrydcase/sidh-torture/csidh

  | TestName         |Go        | C        |
  |------------------|----------|----------|
  |TestSharedSecret  | 57.95774 | 57.91092 |
  |TestKeyGeneration | 62.23614 | 58.12980 |
  |TestSharedSecret  | 55.28988 | 57.23132 |
  |TestKeyGeneration | 61.68745 | 58.66396 |
  |TestSharedSecret  | 63.19408 | 58.64774 |
  |TestKeyGeneration | 62.34022 | 61.62539 |
  |TestSharedSecret  | 62.85453 | 68.74503 |
  |TestKeyGeneration | 52.58518 | 58.40115 |
  |TestSharedSecret  | 50.77081 | 61.91699 |
  |TestKeyGeneration | 59.91843 | 61.09266 |
  |TestSharedSecret  | 59.97962 | 62.98151 |
  |TestKeyGeneration | 64.57525 | 56.22863 |
  |TestSharedSecret  | 56.40521 | 55.77447 |
  |TestKeyGeneration | 67.85850 | 58.52604 |
  |TestSharedSecret  | 60.54290 | 65.14052 |
  |TestKeyGeneration | 65.45766 | 58.42823 |

  On average Go implementation is 2% faster.

* Reset Makefile after adding go.mod
* Remove ``build`` directory
* Simiplifies makefile
* shake: Make xorIn copyOut platform specific

* CTR-DRBG doesn't call "NewCipher" for block encryption
* Changes API of CTR-DRBG, so that read operation implementes io.Reader

Benchmark results:
----------------------
benchmark           old ns/op     new ns/op     delta
BenchmarkInit-4     1118          3579          +220.13%
BenchmarkRead-4     5343          14589         +173.05%

benchmark           old allocs     new allocs     delta
BenchmarkInit-4     15             0              -100.00%
BenchmarkRead-4     67             0              -100.00%

benchmark           old bytes     new bytes     delta
BenchmarkInit-4     1824          0             -100.00%
BenchmarkRead-4     9488          0             -100.00%

* Change license to BSD-3

* SIDH: Multiple developlemnts

* Makefile

* makefile: tools for profiling

* sidh: use SIMD for performing CSWAP

Loads data into 128-bit XMM registers and performs conditional swap.
This is probably less useful for SIDH, but will be useful for cSIDH