ocsp: allocate OpenSSL objects in #initialize{,_copy}

Allocate the underlying OpenSSL object in #initialize{,_copy} instead
of .allocate. Allocating an empty OpenSSL object in .allocate is
wasteful because #initialize{,_copy} cannot always reuse an existing
object and have to allocate a new one.

Some ruby/openssl classes already follow this approach, such as
OpenSSL::PKey::PKey. Let's standardize on it.

Also expand New*() and Set*() macros, which are only used once or twice
and are not very helpful for readability in my opinion.

Author: rhenium