Uses Into<PathBuf> for path in AppendVec::new()

[brooksprumo]

Problem

While reviewing #334, I made a wrong suggestion and noticed that the path parameter in AppendVec::new() et al should be Into<PathBuf> instead of a raw Path or AsRef<Path>.

Summary of Changes

Update AppendVec::new() et al to take the path parameter as Into<PathBuf>, and update callers.

[yhchiang-sol]

LG!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.8%. Comparing base (30eecd6) to head (29f0a3c).
Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##           master     #433     +/-   ##
=========================================
- Coverage    81.9%    81.8%   -0.1%     
=========================================
  Files         840      841      +1     
  Lines      228107   228253    +146     
=========================================
+ Hits       186851   186919     +68     
- Misses      41256    41334     +78     

[HaoranYi]

lgtm