Reuse removed vars in mangler

[boopathi]

Adds a new mangler option - reuse when set to true reuses vars which are removed and not yet used in the current scope.

Test suite:

Now this contains two test suites with identical tests - one uses default option, and the other uses reuse=true.

perf fixes:

• use reuse option in calculating resetnext
• TopLevel option
• don't traverse scope.crawl when not reusing
• don't collect Refs and Bindings when reuse is false

[vigneshshanmugam]

@boopathi This is done for better gzip compression?

[boopathi]

@vigneshshanmugam This test will explain it all -

• Current mangler
• This PR - https://github.com/babel/babili/pull/284/files#diff-82f44e167cd70a768dbf1aa05ee1328eR875

[vigneshshanmugam]

@boopathi cool got it 👍

[boopathi]

Upstream changes - babel/babel#4876