Simplify HeLP_INTERNAL_IsIntVect

[fingolfin]

It also short-circuits, i.e., if any non-int is found it terminates early.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.33%. Comparing base (b2dd66f) to head (7c5d56d).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #27      +/-   ##
==========================================
- Coverage   53.40%   53.33%   -0.08%     
==========================================
  Files           5        5              
  Lines        2451     2447       -4     
==========================================
- Hits         1309     1305       -4     
  Misses       1142     1142              

Files with missing lines	Coverage Δ
lib/HeLP_internal_functions_no_solving.gi	75.34% <100.00%> (-0.17%)	⬇️

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