Add function to entity: entity_access

[moonray]

Paragraphs module uses this function, and it's likely other modules converted from Drupal would use it as well. We should add it to core.

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PR by @jenlampton: backdrop/backdrop#2451

[herbdool]

I took a look at the code and see that tests pass. But best if @quicksketch review.

[quicksketch]

This looks great! I added a lot of feedback to the PR: backdrop/backdrop#2119 (review)

This PR makes some minor API changes as part of the additions. Notably comment_access() and taxonomy_term_access() previously supported an $op of 'edit', but now they are 'update' (matching nodes). We'll need to make a changelog notice for this change once this is completed.

[jenlampton]

Looks like this still needs work, so won't make it into 1.10. Bumping milestone to 1.11.