Add push, pop, shift, & unshift functions.

[FGasper]

This allows code like the following:

foo := []int{1, 2, 3}
lo.Push(&foo, 4)

… which is a bit more concise than Go’s familiar foo = append(foo, 4) idiom.

If the maintainers agree that these are good additions, I’ll add tests before merging.

Thanks!

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 44 lines in your changes missing coverage. Please review.

Project coverage is 92.98%. Comparing base (70b763e) to head (27038f3). Report is 6 commits behind head on master.

Files with missing lines	Patch %	Lines
slice.go	0.00%	44 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #588      +/-   ##
==========================================
- Coverage   94.27%   92.98%   -1.29%     
==========================================
  Files          18       18              
  Lines        3178     3222      +44     
==========================================
  Hits         2996     2996              
- Misses        170      214      +44     
  Partials       12       12              

Flag	Coverage Δ
unittests	92.98% <0.00%> (-1.29%)	:arrow_down:

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[samber]

Please move this work to mutable/ directory.