[billf]

the pre-existing use of core::num::leading_ones and the syn crate require an MSRV beyond that which is currently set.

• 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): set the msrv based on use, dependencies #2 ◀

_{Change managed by git-spice.}