Add merge and copy_merge functionality

[80degreeswest]

Added the following functionality:

• copy_merge: Works similar to copy, but the attribute values are merged together instead of being replaced.
• merge: Merges all attributes between two rules.

[bazel-io]

Can one of the admins verify this patch?

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[80degreeswest]

I signed it!

[googlebot]

CLAs look good, thanks!