Miracl core

[mratsim]

This switches the backend from apache/amcl (which now points to https://github.com/apache/incubator-milagro-crypto-c) to miracl/core which recently changed to Apache license.

Backends and their differences are highlighted here: https://notes.status.im/nim-bls-curve-backends

In particular:

• MIRACL/core is easier to build (no cmake for generating curve-specific code but Python we can easily be translated to Nim)
• The sqrt function is constant-time as required by the IETF Hash-to-curve draft

Changes:

• SQRT does not return if the input was square anymore, this has been separated into a separate QR (quadratic residue) procedure.
• The QR procedure for FP field also sets a hint for a follow-up fast SQRT.
• The internal API of SQRT on FP changed to accept a "hint" parameter.
• setx procedure used in fromHex/fromBytes serialization had to be changed to the new sqrt API. On G1 in particular, the QR procedure is preferred over the jacobi symbol to test if a field element is square so that we can benefit from the fast sqrt hint.

Not done:

• MIRACL/core has a hashToG2 builtin that could be used instead of our own implementation, reducing the audit surface
• ~Cleanup the old submodule~
• Turning on endomorphism acceleration (GLV on G1 and GLS+GLV on G2)

[mratsim]

Bench

Warmup: 0.9007 s, result 224 (displayed to avoid compiler optimizing warmup away)

Compiled with GCC
Optimization level => no optimization: false | release: true | danger: true
Using Milagro with 64-bit limbs
Running on Intel(R) Core(TM) i9-9980XE CPU @ 3.00GHz

⚠️ Cycles measurements are approximate and use the CPU nominal clock: Turbo-Boost and overclocking will skew them.
i.e. a 20% overclock will be about 20% off (assuming no dynamic frequency scaling)

=================================================================================================================

Scalar multiplication G1                                   2845.306 ops/s       351456 ns/op      1054382 cycles
Scalar multiplication G2                                   1006.136 ops/s       993901 ns/op      2981739 cycles
EC add G1                                                947867.299 ops/s         1055 ns/op         3167 cycles
EC add G2                                                312597.687 ops/s         3199 ns/op         9597 cycles
Pairing (Milagro builtin double pairing)                    429.163 ops/s      2330118 ns/op      6990453 cycles
Pairing (Multi-Pairing with delayed Miller and Exp)         436.978 ops/s      2288443 ns/op      6865427 cycles

⚠️ Warning: using draft v5 of IETF Hash-To-Curve (HKDF-based).
           This is an outdated draft.

Hash to G2 (Draft #5)                                      1069.780 ops/s       934772 ns/op      2804354 cycles

Old was: https://github.com/status-im/nim-blscurve/issues/47