SVM: Add doc comments, restrict visibility of some xfaces to crate

[dmakarov]

Problem

Some functions in SVM crate are visible outside the crate, although they're not part of public API.

Summary of Changes

Restrict visibility of functions not used outside SVM crate to the crate only. Add documentation comments. Rearrange the methods of structs according to visibility moving public methods to the beginning of file.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.18310% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 81.8%. Comparing base (ba43f74) to head (3eae916).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master     #136    +/-   ##
========================================
  Coverage    81.8%    81.8%            
========================================
  Files         838      838            
  Lines      226294   226465   +171     
========================================
+ Hits       185271   185416   +145     
- Misses      41023    41049    +26     

[t-nelson]

in the future, please avoid making so many changes in the same pr. at least the test re-organization, docs comments and other refactoring were all valid, independent PRs

[dmakarov]

point taken