Enable parallel execution for make_digits

[mmagician]

Description

On MacBook Air, 4 performance cores, 4 efficiency cores
Before:

MSM for Bls12_381::G1
                        time:   [273.98 ms 290.22 ms 312.05 ms]

After:

MSM for Bls12_381::G1
                        time:   [253.65 ms 255.12 ms 256.76 ms]
                        change: [-18.328% -12.097% -6.8345%] (p = 0.00 < 0.05)
                        Performance has improved.

---

On 16 cores / 32 threads @ 4.2 GHz
Before:

MSM for Bls12_381::G1
                        time:   [126.18 ms 126.47 ms 126.87 ms]

After:

MSM for Bls12_381::G1
                        time:   [114.80 ms 115.20 ms 115.68 ms]
                        change: [-9.3244% -8.9102% -8.4379%] (p = 0.00 < 0.05)
                        Performance has improved.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to GitHub issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the GitHub PR explorer