changes import path from github.com/cloudflare/p751sidh to
github.com/cloudflare/sidh

Using golang.org/x/sys/cpu would be really great, but it makes it
hard to vendor the library with go 1.10.
This patch gets information about CPU capabilities directly from CPUID

* Makes it possible to use p503 in SIDH and SIKE
* Refactors tests so that unit tests in SIKE and SIDH are run for
  each prime field. It adds test data array called 'tdata' which
  describes test parameters for underlying prime field. When new field
  is added, it's enough to simply add new test data to 'tdata' vector in
  order to run all existing tests with new prime field.
* SIKE/p503 is now tested with official test vectors from NIST
  submission

* implements SIKE specified here:
    http://www.sike.org/files/SIDH-spec.pdf
  * methods for both - KEM and PKE - are added
  * adds SIKE specific key variant
  * tests: known answer tests for sike
  * uses cSHKAE from nobs-crypto

* tests: adds continues integration

* Makefile has targets for running tests, benchmarks and code coverage. It also
  contains target for env preparation. In order to run sidh tests
  GOPATH must contain p751toolbox package. I've chosen to manualy
  copy this package to the temporary GOPATH directory. It could also be done
  by calling "go get", but then any commit to both p751toolbox and sidh would need
  to be done in 2 steps.
* .travis.yml calls make and uploads code coverage to Codecov

* move sidh to seperated folder

* sidh: updates algorithm to SIDHv3 and refactoring

* makes an algorithm compatible with Microsoft's SIDHv3
  implementation. This is required to implement SIKE key
  encapsulation mechanism, as specified in PQC NIST submission
  from Nov, 30 2017
* removes SIDHBobPublicKey/SIDHAlicePublicKey/SIDHBobPrivateKey/
  SIDHAlicePrivateKey. Instead PrivateKey and PublicKey structures
  where introduced. Each of this structure stores variant of a key
  A or B. Implementation uses a key variant in order to differentiate
  between 2- and 3-torision groups.
  Main goal of removing "Alice" and "Bob" specific structures is to
  remove code duplication
* Introduces SidhParams: structure to store prime field and SIDH
  domain parameters.
* Refactors public API. Introduces:
    - Functions to generate, import, export keypair
    - DeriveSecret function to create shared secret
    - Supporting functions and types
* Removes code which is not used by implementation anymore, like
  DistortAndDifference(), SecretPoint(), DblAdd(),
  OkeyaSakuraiCoordinateRecovery() and many more. Also tests for those
  functions are removed.
* Adds fixes for key import/export