Introduce dedicated type for processing instruction

[Mingun]

New type has some useful methods:

• target() returns target application to what instruction is targeted. Can be empty string
• content() returns everything else after the target
• attributes() returns content as an Attributes iterator which is useful for some instructions

Closes #650

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 53.73134% with 31 lines in your changes missing coverage. Please review.

Project coverage is 61.78%. Comparing base (5d76174) to head (875a10f).
Report is 37 commits behind head on master.

Files	Patch %	Lines
src/events/mod.rs	41.66%	28 Missing ⚠️
src/writer.rs	0.00%	2 Missing ⚠️
src/writer/async_tokio.rs	0.00%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #756      +/-   ##
==========================================
+ Coverage   61.24%   61.78%   +0.53%     
==========================================
  Files          39       41       +2     
  Lines       16277    16773     +496     
==========================================
+ Hits         9969    10363     +394     
- Misses       6308     6410     +102     

Flag	Coverage Δ
unittests	61.78% <53.73%> (+0.53%)	⬆️

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