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

KeyVariant type needs to be available in API. Instead of using
PrimeFieldId we can simply use uint8

* 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

Private key generation can take advantage of the fact that keyspace for
secret key is (0, 2^x - 1), for some possitivite value of 'x' (see SIKE,
1.3.8). It means that all bytes in the secret key, but the last one, can
take any value between <0x00,0xFF>. Similarily for the last byte, but
generation needs to chop off some bits, to make sure generated value is
an element of a key-space.
Assuming uniform distribution of bytes generated by RNG, secret key is
still chosen uniformly at random, but there is no need to maintain field
specific assembly code.

It makes a little bit more sense to have GeneratePublicKey as a method
of PrivateKey. In this case code doesn't need to check if caller
provided pointer is nil. Object was created by NewPrivateKey(), so it
code can assume object was correctly initialized.
The old GeneratePublicKey was returning an error when caller provided
pointer was nil. As this possibility is now removed, method doesn't
return error anymore.

* Code is much slower than x86 specialized implementation
* Cross checked on ARMv8 (32-bit)

* 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