Make AST classes more explicitly value objects

[mattwynne]

Criteria:

1. immutable
2. equality comparison works (by location?)

See 2e85fcb#commitcomment-10458359

[mattwynne]

See also #84

[brasmusson]

this attr_accessor in background.rb should be possible to change to a attr_reader (the attributes are set in the constructor). What about this attr_accessor in doc_string.rb is the file attribute ever used? It is not set in the constructor, and I can remove the line from the file without neither the cucumber-ruby-core test nor the cucumber test fails.

[mattwynne]

@brasmusson if you can remove it without breaking tests, I'm happy to rip it out.

[brasmusson]

All attr_accessor declarations are removed from the ast classes (044cca8)

[stale]

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[stale]

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[lock]

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