Supply location info to class_eval

[taichi-ishitani]

Currently class_eval has no location info so caller location of FallbackContextProxy#method_missing is not understandable.
(see following example.)
This PR is fix this problem.

For example:

Sample code:

require 'docile'

class Foo
  def bar
    puts caller_locations
  end
end

Docile.dsl_eval(Foo.new) { bar }

Current caller locations:

(eval):3:in `method_missing'
test.rb:9:in `block in <main>'
/opt/rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/docile-1.3.3/lib/docile/execution.rb:26:in `instance_exec'
/opt/rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/docile-1.3.3/lib/docile/execution.rb:26:in `exec_in_proxy_context'
test.rb:9:in `<main>'

Better and expected caller locations:

/home/taichi/workspace/ruby_etc/docile/lib/docile/fallback_context_proxy.rb:94:in `method_missing'
test.rb:9:in `block in <main>'
/home/taichi/workspace/ruby_etc/docile/lib/docile/execution.rb:26:in `instance_exec'
/home/taichi/workspace/ruby_etc/docile/lib/docile/execution.rb:26:in `exec_in_proxy_context'
/home/taichi/workspace/ruby_etc/docile/lib/docile.rb:44:in `dsl_eval'
test.rb:9:in `<main>'

[codecov-io]

Codecov Report

Merging #50 (3a9a18a) into master (f065fc3) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #50   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            6         6           
  Lines           82        82           
=========================================
  Hits            82        82           

Impacted Files	Coverage Δ
lib/docile/fallback_context_proxy.rb	100.00% <100.00%> (ø)

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[taichi-ishitani]

#51 includes my PR so I close it.