Add .apply_fn() for chain and chain factory

[fafhrd91]

No description provided.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (594bf0a) 90.95% compared to head (1faad84) 90.97%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   90.95%   90.97%   +0.01%     
==========================================
  Files         189      189              
  Lines       31014    31059      +45     
==========================================
+ Hits        28210    28255      +45     
  Misses       2804     2804              

Files Changed	Coverage Δ
ntex-service/src/apply.rs	98.23% <100.00%> (+0.31%)	⬆️
ntex-service/src/chain.rs	100.00% <100.00%> (ø)

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