Add `multiset_permutations` method that doesn't require length

[oscardssmith]

in analogy to the same case for regular permuations

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 96.41%. Comparing base (a303e88) to head (6afa6f0).

:exclamation: Current head 6afa6f0 differs from pull request most recent head 2813a19. Consider uploading reports for the commit 2813a19 to get more accurate results

Files	Patch %	Lines
src/permutations.jl	0.00%	4 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #164      +/-   ##
==========================================
- Coverage   96.89%   96.41%   -0.48%     
==========================================
  Files           8        8              
  Lines         804      808       +4     
==========================================
  Hits          779      779              
- Misses         25       29       +4     

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[inkydragon]

Maybe add some tests or doctests

[oscardssmith]

test added.

[oscardssmith]

should this be merged?