Traditionally, method coverage measurement was implemented by inserting
`trace2` instruction to the head of method iseq.  So, it just measured
methods defined by `def` keyword.

This commit drastically changes the measuring mechanism of method
coverage; at `RUBY_EVENT_CALL`, it keeps a hash from rb_method_entry_t*
to runs (i.e., it counts the runs per method entry), and at
`Coverage.result`, it creates the result hash by enumerating all
`rb_method_entry_t*` objects (by `ObjectSpace.each_object`).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61023 b2dd03c8-39d4-4d8f-98ff-823fe69b080e