Improve bcodec encode performance

[zzau13]

Remove repeat calls to reverve method. Stop treating itemsince it has the assured drop. Add the inline attribute because the resulting body is just one line.

The same as ntex-rs/ntex#22

[codecov]

Codecov Report

Merging #157 into master will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
- Coverage   60.50%   60.50%   -0.01%     
==========================================
  Files          74       74              
  Lines        4791     4790       -1     
==========================================
- Hits         2899     2898       -1     
  Misses       1892     1892              

Impacted Files	Coverage Δ
actix-codec/src/bcodec.rs	37.50% <100.00%> (-6.95%)	⬇️

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[robjtede]

nice catch 👍

Can you put a mention of the perf improvement in changelog please.