Use public prepend, include, and extend methods

[marcotc]

This uses the public version of Module#prepend and Module#include.

#prepend and #include were private in 2.0.

Also, this PR cleans up usages of Object#extend that used the send(:extend) syntax, instead of #extend. This method was public in Ruby 2.0, but we were likely conservative with its call syntax due to its apparent proximity to the non-public #prepend and #include.

Ruby 2.0 deprecation checklist

• Remove 2.0 from CI
• Remove 2.0-specific code paths
• Update to 2.1+ standards: https://github.com/ruby/ruby/blob/v2_1_0/NEWS
  • Module#include and Module#prepend are now public methods.
  • Exception#cause
  • Array#to_h converts an array of key-value pairs into a Hash.
  • Kernel#singleton_method
  • Process.clock_gettime
  • Enumerable#to_h converts a list of key-value pairs into a Hash.
  • Now the default values of keyword arguments can be omitted. Those “required keyword arguments” need giving explicitly at the call time.
• Update documentation

[codecov-commenter]

Codecov Report

Merging #1509 (f9245ef) into master (88d321b) will not change coverage.
The diff coverage is 98.42%.

@@           Coverage Diff           @@
##           master    #1509   +/-   ##
=======================================
  Coverage   98.22%   98.22%           
=======================================
  Files         862      862           
  Lines       41686    41686           
=======================================
  Hits        40945    40945           
  Misses        741      741           

Impacted Files	Coverage Δ
lib/ddtrace/contrib/faraday/patcher.rb	92.30% <50.00%> (ø)
spec/ddtrace/profiling/ext/cthread_spec.rb	90.00% <85.71%> (ø)
lib/ddtrace/configuration/base.rb	97.67% <100.00%> (ø)
lib/ddtrace/configuration/options.rb	100.00% <100.00%> (ø)
lib/ddtrace/contrib/action_cable/event.rb	100.00% <100.00%> (ø)
...e/contrib/action_pack/action_controller/patcher.rb	100.00% <100.00%> (ø)
lib/ddtrace/contrib/action_view/event.rb	100.00% <100.00%> (ø)
lib/ddtrace/contrib/action_view/patcher.rb	96.29% <100.00%> (ø)
.../ddtrace/contrib/active_model_serializers/event.rb	100.00% <100.00%> (ø)
lib/ddtrace/contrib/active_record/event.rb	100.00% <100.00%> (ø)
... and 67 more

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