Iteratorify some tableaux

[joschmitt]

Turn semistandard_tableaux(shape) and standard_tableaux into an actual iterator internally.
Also adds tests/fixes for different Integer types. ZZRingElem doesn't work in general because the entries are often used as indices.

The only function left, which still first fills an array internally and then iterates the entries, is semistandard_tableaux(shape, weight).
This function is unfortunately implemented recursively, so it would need to be reimplemented completely. There is no reference for the used algorithm given; I assume it was implemented by a Bachelor's student in JuLie originally. @ulthiel Is there a reference for this algorithm?

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 8 lines in your changes missing coverage. Please review.

Project coverage is 83.87%. Comparing base (5acca40) to head (d177f19).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3887   +/-   ##
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  Coverage   83.87%   83.87%           
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  Files         589      589           
  Lines       80864    80940   +76     
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+ Hits        67823    67892   +69     
- Misses      13041    13048    +7     

Files	Coverage Δ
...rc/Combinatorics/EnumerativeCombinatorics/types.jl	100.00% <100.00%> (ø)
...Combinatorics/EnumerativeCombinatorics/tableaux.jl	96.31% <94.90%> (-1.03%)	⬇️

[JohnAAbbott]

The doc for YTs mentions that the tableaux entries are "usually" (positive) integers in the range 1..n where n is the number of cells in the YT. This suggests two possible modifications to the UI:

1. make the constructor for a YT accept only integers in the range 1..n; offer a boolean kwarg which allows positive entries from a wider range -- I have asked a few people, and no-one seemed ever to have seen non-positive entries in a YT
2. insist that entries are of type Int: in ptic, disallow ZZRingElem; allowing Int8 or other "small machine integer types" seems to bring no benefit, now that iterator are genuine iterators (previously Int8 brought a minor advantage because less memory was needed for storing the full list)

What do you think about these suggestions?

[fingolfin]

Looks good to me

[joschmitt]

I copied the open questions to #3850.