Define move semantics for peg::ast<> and return by value

[wravery]

Breaking API change, requires bumping the major version for the next release. Might want to release a minor version update with the subscription argument filtering and other bug fixes first unless either of those introduces a breaking change.

[wravery]

I think each of those might require a breaking API change. To bubble up field errors I'll need to either change the return type of Object::resolve or add an out-param to append to the errors collection in addition to returning the field value, and it should also be done with a future.

[wravery]

The subscription argument filtering might not introduce a breaking change from the 1.0.* releases. I think I can get by with another deliver override which passes an argument comparator callback. The deliver override I added in #26 can pass a lambda with the current exact match logic for backwards compatibility and default comparisons.