Refuse opaque closures.

[maleadt]

These are objects containing a CPU pointer. They would also fail the mutability check below, but let's be a little more verbose.

[codecov]

Codecov Report

Patch coverage: 33.33% and project coverage change: -1.25 ⚠️

Comparison is base (5de5a5c) 87.06% compared to head (ac74605) 85.81%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #447      +/-   ##
==========================================
- Coverage   87.06%   85.81%   -1.25%     
==========================================
  Files          24       24              
  Lines        2961     2954       -7     
==========================================
- Hits         2578     2535      -43     
- Misses        383      419      +36     

Impacted Files	Coverage Δ
src/validation.jl	96.27% <33.33%> (-1.20%)	⬇️

... and 8 files with indirect coverage changes

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