rtp: expose serial generator as a function, with helpers

[adamdecaf]

I've adapted the two below PR's into a more general approach and added it for every serial function.

See #59 and #60

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 92.85714% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 84.65%. Comparing base (8d3bebe) to head (c1c66b0).

❗ Current head c1c66b0 differs from pull request most recent head c8db072. Consider uploading reports for the commit c8db072 to get more accurate results

Files	Patch %	Lines
pkg/rtp/serials.go	87.50%	1 Missing and 1 partial ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master      #61      +/-   ##
==========================================
+ Coverage   84.42%   84.65%   +0.22%     
==========================================
  Files          13       13              
  Lines         199      215      +16     
==========================================
+ Hits          168      182      +14     
- Misses         26       27       +1     
- Partials        5        6       +1     

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