scalar: introduce and use secp256k1_{read,write}_be64 helpers #1350

1. 
   theStack commented at 0:01 am on June 17, 2023: contributor

   This is a simple follow-up to #1339, as suggested in comment #1339 (comment).
2. scalar: use newly introduced `secp256k1_{read,write}_be64` helpers (4x64 impl.) 740528caad
3. tests: add tests for `secp256k1_{read,write}_be64`

   This can be reviewed with `--ignore-all-space` (or `-w`), to ignore
   already existing code that was only indented.

   7067ee54b4
4. 
   stratospher commented at 4:17 am on June 18, 2023: contributor

   ACK 7067ee5. Functions look simpler to understand! Cross checked that r->d[0], r->d[1], r->d[2], r->d[3] values match for the same input on master and this PR.
5. real-or-random approved
6. 
   real-or-random commented at 6:32 pm on June 18, 2023: contributor

   utACK 7067ee54b4206c26b382980f3c20b5fa0262a23a
7. real-or-random merged this on Jun 18, 2023
8. real-or-random closed this on Jun 18, 2023

   ---

9. theStack deleted the branch on Jun 18, 2023
10. sipa referenced this in commit 901336eee7 on Jun 21, 2023
11. vmta referenced this in commit 8f03457eed on Jul 1, 2023
12. hebasto referenced this in commit 270d2b37b8 on Jul 21, 2023
13. jonasnick cross-referenced this on Jul 26, 2023 from issue Upstream PRs 1314, 1317, 1318, 1316, 1327, 1310, 1328, 1333, 1330, 1334, 1337, 1341, 1339, 1350, 1349, 1338, 1129, 1347, 1336, 1295, 1354, 1355, 1356 by jonasnick

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