fix(msrv): dependencies require v1.61, 2021 edition

[billf]

due to use of std::num functions and the syn dependency, we must advance the MSRV.

• fix(msrv): num::leading_{one,zero}s requires v1.46
• fix(msrv): implicit dependency syn requires 1.61
• chore(msrv): rust v1.61 allows 2021 edition

[billf]

This change is part of the following stack:

• chore(fmt): rustfmt before any submission #1
  • fix(msrv): dependencies require v1.61, 2021 edition #3 ◀

_{Change managed by git-spice.}