Clean up public API surface

[oremanj]

• Underscore-prefix module names
• Hide internal module names from public class/function metadata
• Fix circular dependencies
• Add actual deprecation warnings when deprecated entities are used
• Substantially improve documentation

Fixes #64

[codecov[bot]]

Codecov Report

Merging #69 into master will increase coverage by 5.67%. The diff coverage is 87.43%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   72.66%   78.33%   +5.67%     
==========================================
  Files          11       11              
  Lines        1284     1288       +4     
  Branches      177      180       +3     
==========================================
+ Hits          933     1009      +76     
+ Misses        281      204      -77     
- Partials       70       75       +5     

Impacted Files	Coverage Δ
trio_asyncio/_deprecate.py	80.55% <0.00%> (ø)