More robust constructors

[jakobnissen]

• Use cconvert instead of accessing the internal .ref field of Array. This is still not entirely stable API, but hopefully better
• Do not use inbounds for substring of AbstractString, because the implementor of the AbstractString may lie about the codeunits, and I'm not confident the substring constructor will detect it
• Use ncodeunits instead of reading the ncodeunit fields of substrings
• Be slightly more conservative in what subarrays the constructor accepts.

The subarray changes are technically breaking, but since the previous behaviour was buggy, AND the bug would throw an internal error, AND it's unlikely anyone relied on it, I'll add it in a minor release

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.82%. Comparing base (0a56a55) to head (2412aae).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #18      +/-   ##
==========================================
- Coverage   96.83%   96.82%   -0.02%     
==========================================
  Files           5        5              
  Lines         253      252       -1     
==========================================
- Hits          245      244       -1     
  Misses          8        8              

Flag	Coverage Δ
unittests	96.82% <100.00%> (-0.02%)	⬇️

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