:new: [C++20] Concepts generation from type erased interface declaration

[krzysztof-jusiak]

Problem:

• Type erased interface declaration might be used to generate concept type.

Solution:

• Add var to constraint variables.
• Add conceptify to constraint functions.

[codecov[bot]]

Codecov Report

Merging #5 into master will increase coverage by 0.04%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #5      +/-   ##
==========================================
+ Coverage   99.22%   99.27%   +0.04%     
==========================================
  Files          13       14       +1     
  Lines         389      411      +22     
==========================================
+ Hits          386      408      +22     
  Misses          3        3

Impacted Files	Coverage Δ
include/te.hpp	100% <ø> (ø)	:arrow_up:
test/te.cpp	100% <100%> (ø)	:arrow_up:
example/concepts.cpp	100% <100%> (ø)

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