Consider JS evaluation costs in efficiency audits

currently, the byte efficieny audits treat every byte as equal, only measuring the network cost. However, JavaScript bytes are also costly in terms of CPU.

For example:

The `unused-javascript` audit points out code that is not executed - but the code is still *parsed*.

The (in progress) `bundle-duplication` audit points out extra code that is *parsed* and *compiled* (correlating w/ `unused-javascript` would be needed to confirm that last bit, but it may be OK to assume all duplicated code is compiled).

Needs much investigation.