Fix UB in hash implementation

[Enchufa2]

As reported by CRAN:

nn2poly_types.h:25:27: runtime error: signed integer overflow: 73243494 * 73244475 cannot be represented in type 'int'
      #0 0x7b59bd7d8a34 in TermHash::operator()(std::__1::vector<int, std::__1::allocator<int>> const&) const /__w/nn2poly/nn2poly/check/nn2poly.Rcheck/00_pkg_src/nn2poly/src/./nn2poly_types.h:25:27

etc. Some other minor fixes here and there now that we're at it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.92%. Comparing base (4cdc4c1) to head (a655bfb).

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #84      +/-   ##
==========================================
+ Coverage   83.90%   83.92%   +0.02%     
==========================================
  Files          18       18              
  Lines         758      759       +1     
==========================================
+ Hits          636      637       +1     
  Misses        122      122              

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[Enchufa2]

New checks are clean, merging.