Fix top_const_ref parsing

[dduugg]

Previously, top_const_ref nodes, e.g. ::Foo, would have the leading :: removed.

This can be seen at https://rubydoc.brew.sh/OS/Mac.html#latest_sdk_version-class_method
The return type links to https://rubydoc.brew.sh/OS/Mac/Version.html, rather than https://rubydoc.brew.sh/Version.html

(This PR also includes misc. cleanup in the sig parser)

[codecov]

Codecov Report

Merging #63 (562ed81) into master (4620a0b) will decrease coverage by 0.49%.
The diff coverage is 91.17%.

@@            Coverage Diff             @@
##           master      #63      +/-   ##
==========================================
- Coverage   99.31%   98.82%   -0.50%     
==========================================
  Files          16       16              
  Lines         588      595       +7     
==========================================
+ Hits          584      588       +4     
- Misses          4        7       +3     

Impacted Files	Coverage Δ
lib/yard-sorbet/sig_to_yard.rb	90.90% <90.90%> (-3.38%)	⬇️
spec/yard_sorbet/handlers/sig_handler_spec.rb	100.00% <100.00%> (ø)

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