Fr: Finite Field parametrized by the curve order (#115)

* Introduce Fr type: finite field over curve order. Need workaround for nim-lang/Nim#16774

* Split curve properties into core and derived

* Attach field properties to an instantiated field instead of the curve enum

* Workaround nim-lang/Nim#14021, yet another "working with types in macros" is difficult nim-lang/RFCs#44

* Implement finite field over prime order of a curve subgroup

* skip OpenSSL tests on windows

benchmarks/bench_fields_template.nim

@@ -105,7 +105,7 @@ proc invEuclidBench*(T: typedesc, iters: int) =
 
 proc invPowFermatBench*(T: typedesc, iters: int) =
   let x = rng.random_unsafe(T)
-  const exponent = T.C.getInvModExponent()
+  const exponent = T.getInvModExponent()
   bench("Inversion via exponentiation p-2 (Little Fermat)", T, iters):
     var r = x
     r.powUnsafeExponent(exponent)

constantine.nimble

@@ -148,7 +148,12 @@ const testDesc: seq[tuple[path: string, useGMP: bool]] = @[
   ("tests/t_pairing_bls12_381_optate.nim", false),
 
   # Hashing vs OpenSSL
-  ("tests/t_hash_sha256_vs_openssl.nim", true),
+  # ----------------------------------------------------------
+  ("tests/t_hash_sha256_vs_openssl.nim", true), # skip OpenSSL tests on Windows
+
+  # Prime order fields
+  # ----------------------------------------------------------
+  ("tests/t_fr.nim", false),
 ]
 
 # For temporary (hopefully) investigation that can only be reproduced in CI
@@ -264,7 +269,7 @@ proc buildAllBenches() =
   buildBench("bench_pairing_bn254_snarks")
   buildBench("bench_sha256")
   echo "All benchmarks compile successfully."
-  
+
 # Tasks
 # ----------------------------------------------------------------