Add FilterByType

[Syuparn]

added FilterByType function. This can skip type assertion after filtering.

Closes #339

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (2a74434) 99.31% compared to head (8c7af7b) 99.31%.

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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #340   +/-   ##
=======================================
  Coverage   99.31%   99.31%           
=======================================
  Files          15       15           
  Lines        2037     2045    +8     
=======================================
+ Hits         2023     2031    +8     
  Misses         13       13           
  Partials        1        1           

Flag	Coverage Δ
unittests	99.31% <100.00%> (+<0.01%)	:arrow_up:

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
slice.go	99.11% <100.00%> (+0.02%)	:arrow_up:

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