Revert https://github.com/ruby/ruby/pull/2486

This reverts commits: 10d6a3aca7 8ba48c1b85 fba8627dc1 dd883de5ba
6c6a25feca 167e6b48f1 7cb96d41a5 3207979278 595b3c4fdd 1521f7cf89
c11c5e69ac cf33608203 3632a812c0 f56506be0d 86427a3219 .

The reason for the revert is that we observe ABA problem around
inline method cache.  When a cache misshits, we search for a
method entry.  And if the entry is identical to what was cached
before, we reuse the cache.  But the commits we are reverting here
introduced situations where a method entry is freed, then the
identical memory region is used for another method entry.  An
inline method cache cannot detect that ABA.

Here is a code that reproduce such situation:

```ruby
require 'prime'

class << Integer
  alias org_sqrt sqrt
  def sqrt(n)
    raise
  end

  GC.stress = true
  Prime.each(7*37){} rescue nil # <- Here we populate CC
  class << Object.new; end

  # These adjacent remove-then-alias maneuver
  # frees a method entry, then immediately
  # reuses it for another.
  remove_method :sqrt
  alias sqrt org_sqrt
end

Prime.each(7*37).to_a # <- SEGV
```

ext/coverage/coverage.c

@@ -123,7 +123,7 @@ method_coverage_i(void *vstart, void *vend, size_t stride, void *data)
 
     for (v = (VALUE)vstart; v != (VALUE)vend; v += stride) {
 	if (RB_TYPE_P(v, T_IMEMO) && imemo_type(v) == imemo_ment) {
-            const rb_method_entry_t *me = (const rb_method_entry_t *) v;
+	    const rb_method_entry_t *me = (rb_method_entry_t *) v;
 	    VALUE path, first_lineno, first_column, last_lineno, last_column;
 	    VALUE data[5], ncoverage, methods;
 	    VALUE methods_id = ID2SYM(rb_intern("methods"));