Add ReplacePlaceholders

[bernhardmgruber]

This is a Mp11 based meta function that replaces placeholders inside a template instantiation.

E.g. llama::ReplacePlaceholders<mp_list<mp_list<_2, _1>, _2, mp_list<_2, mp_list<_3>>>, A, B, C> is an alias to mp_list<mp_list<B, A>, B, mp_list<B, mp_list<C>>>>.

[codecov]

Codecov Report

Merging #451 (af0f95c) into develop (06c7f42) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #451   +/-   ##
========================================
  Coverage    99.21%   99.21%           
========================================
  Files           58       59    +1     
  Lines         5198     5233   +35     
========================================
+ Hits          5157     5192   +35     
  Misses          41       41           

Impacted Files	Coverage Δ
tests/meta.cpp	100.00% <100.00%> (ø)
tests/core.cpp	100.00% <0.00%> (ø)
include/llama/Core.hpp	94.59% <0.00%> (+2.00%)	⬆️

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