feat: make BitField::new const

[Stebalien]

This won't actually improve performance, but may help with some ergonomics.

[codecov-commenter]

Codecov Report

Merging #850 (9af4004) into master (3ad5b90) will decrease coverage by 6.19%.
The diff coverage is 100.00%.

❗ Current head 9af4004 differs from pull request most recent head 0ae2f43. Consider uploading reports for the commit 0ae2f43 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #850      +/-   ##
==========================================
- Coverage   74.97%   68.78%   -6.19%     
==========================================
  Files         148      149       +1     
  Lines       14281    16758    +2477     
==========================================
+ Hits        10707    11527     +820     
- Misses       3574     5231    +1657     

Impacted Files	Coverage Δ
ipld/bitfield/src/lib.rs	78.53% <100.00%> (+0.45%)	⬆️

... and 88 files with indirect coverage changes

[Stebalien]

Guess that isn't stable yet.

[mriise]

for posterity, rustlang tracking issue for this: rust-lang/rust#71835

[Stebalien]

cleaning up old branches...