improvements to with_prefix_location

[samuelcolvin]

as suggested by @afetisov in #35

Profiled this and it seems to make a 1% improvement in the case of lots of errors.

[codecov]

Codecov Report

Merging #99 (b5ccad9) into main (f26af27) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #99   +/-   ##
=======================================
  Coverage   95.87%   95.88%           
=======================================
  Files          40       40           
  Lines        3491     3497    +6     
  Branches       28       28           
=======================================
+ Hits         3347     3353    +6     
  Misses        144      144           

Impacted Files	Coverage Δ
src/errors/line_error.rs	97.53% <100.00%> (-0.18%)	⬇️
src/input/generics.rs	96.42% <100.00%> (+0.20%)	⬆️
src/input/to_loc_item.rs	95.45% <100.00%> (+0.71%)	⬆️
src/validators/dict.rs	94.94% <100.00%> (+0.05%)	⬆️
src/validators/model.rs	95.07% <100.00%> (-0.03%)	⬇️
src/validators/tuple.rs	98.51% <100.00%> (+0.03%)	⬆️
src/validators/union.rs	97.22% <100.00%> (+0.25%)	⬆️

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[samuelcolvin]

merging this, but happy to accept more changes if suggested.