pkey: make OpenSSL::PKey::EC::Group wrap an EC_GROUP directlytopic/pkey-ec-unlink

As done for EC::Point, remove ossl_ec_group struct. This contains a
breaking change. Modifications to an EC::Group returned by EC#group
no longer affects the EC object unless set to the key explicitly using
EC#group=. This is the common behavior in Ruby/OpenSSL, including other
getter methods of EC such as EC#public_key.

EC#group currently returns a EC::Group linked with the key, i.e. the
EC::Group object holds a reference to an EC_GROUP that the EC_KEY owns.
We use some ugly workaround - the ossl_ec_group struct has a flag
'dont_free' that indicates we must not free the EC_GROUP. But it is
still not possible to control OpenSSL of free'ing the EC_GROUP, so,
for example, the following code behaves strangely:

  ec = OpenSSL::PKey::EC.generate("prime256v1")
  group = ec.group
  p group.curve_name #=> "prime256v1"
  ec.group = OpenSSL::PKey::EC::Group.new("prime256v1")
  p group.curve_name #=> nil

1 files changed, 12 insertions, 6 deletions

diff --git a/History.md b/History.md
index da014a54..e7434521 100644
--- a/History.md
+++ b/History.md
@@ -56,13 +56,19 @@ Notable changes
   - OpenSSL::OCSP::BasicResponse#add_status accepts absolute times. They used to
     accept only relative seconds from the current time.
 
-* OpenSSL::PKey::EC follows the general PKey interface.
-  [[Bug #6567]](https://bugs.ruby-lang.org/issues/6567)
+* OpenSSL::PKey
+
+  - OpenSSL::PKey::EC follows the general PKey interface.
+    [[Bug #6567]](https://bugs.ruby-lang.org/issues/6567)
+
+  - OpenSSL::PKey.read raises OpenSSL::PKey::PKeyError instead of ArgumentError
+    for consistency with OpenSSL::PKey::{DH,DSA,RSA,EC}#new.
+    [[Bug #11774]](https://bugs.ruby-lang.org/issues/11774), 
+    [[GH ruby/openssl#55]](https://github.com/ruby/openssl/pull/55)
 
-* OpenSSL::PKey.read raises OpenSSL::PKey::PKeyError instead of ArgumentError
-  for consistency with OpenSSL::PKey::{DH,DSA,RSA,EC}#new.
-  [[Bug #11774]](https://bugs.ruby-lang.org/issues/11774), 
-  [[GH ruby/openssl#55]](https://github.com/ruby/openssl/pull/55)
+  - OpenSSL::PKey::EC::Group retrieved by OpenSSL::PKey::EC#group is no longer
+    linked with the EC key. Modifications to the EC::Group have no effect on the
+    key. [[GH ruby/openssl#71]](https://github.com/ruby/openssl/pull/71)
 
 * OpenSSL::SSL