Add Order constants to all curve implementations

[tarcieri]

Allows checking that a scalar field element is in range generically without a curve arithmetic implementation.

[codecov-commenter]

Codecov Report

Merging #328 (9d8a290) into master (571fd7e) will not change coverage.
The diff coverage is n/a.

❗ Current head 9d8a290 differs from pull request most recent head fe235d2. Consider uploading reports for the commit fe235d2 to get more accurate results

@@           Coverage Diff           @@
##           master     #328   +/-   ##
=======================================
  Coverage   58.43%   58.43%           
=======================================
  Files          35       35           
  Lines        3950     3950           
=======================================
  Hits         2308     2308           
  Misses       1642     1642           

Impacted Files	Coverage Δ
bp256/src/r1.rs	0.00% <ø> (ø)
bp256/src/t1.rs	0.00% <ø> (ø)
bp384/src/r1.rs	0.00% <ø> (ø)
bp384/src/t1.rs	0.00% <ø> (ø)
k256/src/arithmetic/scalar/scalar_4x64.rs	0.00% <ø> (ø)
k256/src/lib.rs	0.00% <ø> (ø)
p256/src/lib.rs	0.00% <ø> (ø)
p384/src/lib.rs	0.00% <ø> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 571fd7e...fe235d2. Read the comment docs.