Add tests for getting constants from instances.
Implement getting constants from objects.
Update documentation for constant()
Fix doc block for twig_constant.
merged branch markstory/object-constants (PR #961)

This PR was merged into the master branch.

Commits
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8bb953c Fix doc block for twig_constant.
c7dafb7 Update documentation for constant()
cb921ce Implement getting constants from objects.
998b367 Add tests for getting constants from instances.

Discussion
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Object constants

Implement the changes discussed in #954.
tweaked previous merge