Use threads for parallelization

* The parallelization runner almost immediately calls a fork and gets
  blocked on waitpid() releasing the GIL.
* Threads have lower resource usage than processes
* There is a chance mutant will get faster on smaller machines and might
  scale better on bigger.

lib/mutant.rb

@@ -221,7 +221,7 @@ module Mutant
       isolation:         Mutant::Isolation::Fork,
       reporter:          Reporter::CLI.build($stdout),
       zombie:            false,
-      processes:         Mutant.ci? ? CI_DEFAULT_PROCESSOR_COUNT : Parallel.processor_count,
+      jobs:              Mutant.ci? ? CI_DEFAULT_PROCESSOR_COUNT : Parallel.processor_count,
       expected_coverage: 100.0
     )
   end # Config

lib/mutant/cli.rb

@@ -112,8 +112,8 @@ module Mutant
       opts.on('-r', '--require NAME', 'Require file with NAME') do |name|
         add(:requires, name)
       end
-      opts.on('-j', '--jobs NUMBER', 'Number of kill processes. Defaults to number of processors.') do |number|
-        update(processes: Integer(number))
+      opts.on('-j', '--jobs NUMBER', 'Number of kill jobs. Defaults to number of processors.') do |number|
+        update(jobs: Integer(number))
       end
     end
 

lib/mutant/config.rb

@@ -10,7 +10,7 @@ module Mutant
       :reporter,
       :isolation,
       :fail_fast,
-      :processes,
+      :jobs,
       :zombie,
       :expected_coverage
     )

lib/mutant/reporter/cli/printer.rb

@@ -177,7 +177,7 @@ module Mutant
             info 'Matcher:            %s',      object.matcher_config.inspect
             info 'Integration:        %s',      object.integration.name
             info 'Expect Coverage:    %0.2f%%', object.expected_coverage.inspect
-            info 'Processes:          %d',      object.processes
+            info 'Jobs:               %d',      object.jobs
             info 'Includes:           %s',      object.includes.inspect
             info 'Requires:           %s',      object.requires.inspect
             self

lib/mutant/runner.rb

@@ -46,9 +46,9 @@ module Mutant
     def run
       Parallel.map(
         @mutations,
-        in_processes: config.processes,
-        finish:       method(:finish),
-        start:        method(:start),
+        in_threads: config.jobs,
+        finish:     method(:finish),
+        start:      method(:start),
         &method(:run_mutation)
       )
     end