Make introspection XML better diffable for docs

[mvidner]

(This is useful for agama-project/agama#415)

That is, make separate closing tags, so that our output can be easier edited (and diffed line by line) to add XML documentation.

This is a small performance penalty for clients that do runtime introspection.
To avoid introspection, call ProxyObjectInterface#define_method, see examples/no-introspect/tracker-test.rb
(TODO: make it easier, all the way from the top)

[coveralls]

Coverage: 95.324%. Remained the same when pulling 015517b on diffable-introspection into dae55db on master.

[mvidner]

In the end this is not needed, we perform more complex diffing anyway