bip32: use const panic for Prefix::from_parts_unchecked

[tony-iqlusion]

Now that we're MSRV 1.57, const panic can be used because it was stabilized in that release.

[codecov-commenter]

Codecov Report

Merging #995 (dd97131) into main (725ac32) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #995   +/-   ##
=======================================
  Coverage   62.78%   62.78%           
=======================================
  Files          64       64           
  Lines        2147     2147           
=======================================
  Hits         1348     1348           
  Misses        799      799           

Impacted Files	Coverage Δ
bip32/src/prefix.rs	64.70% <100.00%> (ø)

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