guard against trait type piracy in a dependent package

[nsajko]

Prevent dependent packages from commiting type piracy easily and unintentionally when defining an AbstractTrees trait for all subtypes of a type. Specifically: the bottom type, Union{}, is a subtype of each type, so add a method for the bottom type for each AbstractTrees trait. This method will take precedence over sane methods in dependent packages, thus preventing spurious type piracy.

[codecov-commenter]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 71.49%. Comparing base (4fe5615) to head (2c0f73b).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #157      +/-   ##
==========================================
+ Coverage   71.16%   71.49%   +0.32%     
==========================================
  Files           8        8              
  Lines         437      442       +5     
==========================================
+ Hits          311      316       +5     
  Misses        126      126              

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[nsajko]

This is how Base does the same thing, for eltype, IteratorSize and IteratorEltype:

• https://github.com/JuliaLang/julia/blob/8567a3a10f4b746b91bf406bfe3171c3399aed8d/base/abstractarray.jl#L240-L240

• https://github.com/JuliaLang/julia/blob/8567a3a10f4b746b91bf406bfe3171c3399aed8d/base/generator.jl#L96-L96

• https://github.com/JuliaLang/julia/blob/8567a3a10f4b746b91bf406bfe3171c3399aed8d/base/generator.jl#L130-L130

The slurp stuff is presumably there to help inference somehow. I don't think it's important to have that.