refactor: add support for go1.16's WalkDir

[darkliquid]

Add support for using the faster WalkDir method when building against
go1.16.

This hopefully addresses the concerns with #15 by making this backwards compatible.

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 10 lines in your changes missing coverage. Please review.
✅ Project coverage is 66.53%. Comparing base (0b7b89d) to head (853d54f).
⚠️ Report is 16 commits behind head on master.

Files with missing lines	Patch %	Lines
finder_dirwalk.go	58.33%	3 Missing and 2 partials ⚠️
finder_walk.go	58.33%	3 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           master      #22       +/-   ##
===========================================
+ Coverage   10.34%   66.53%   +56.18%     
===========================================
  Files           4        6        +2     
  Lines         232      248       +16     
===========================================
+ Hits           24      165      +141     
+ Misses        208       71      -137     
- Partials        0       12       +12     

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