ext/coverage/: add the oneshot mode

This patch introduces "oneshot_lines" mode for `Coverage.start`, which
checks "whether each line was executed at least once or not", instead of
"how many times each line was executed".  A hook for each line is fired
at most once, and after it is fired, the hook flag was removed; it runs
with zero overhead.

See [Feature #15022] in detail.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@65195 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

ext/coverage/lib/coverage.rb

@@ -0,0 +1,14 @@
+require "coverage.so"
+
+module Coverage
+  def self.line_stub(file)
+    lines = File.foreach(file).map { nil }
+    iseqs = [RubyVM::InstructionSequence.compile_file(file)]
+    until iseqs.empty?
+      iseq = iseqs.pop
+      iseq.trace_points.each {|n, _| lines[n - 1] = 0 }
+      iseq.each_child {|child| iseqs << child }
+    end
+    lines
+  end
+end