duplicated helpers

https://github.com/bublejs/buble/blob/ecc42f5685810a966bd53b77866028f5322ac7ac/src/index.js#L95

Each run a new Program object is created and causes helpers like `objectWithoutProperties` to be duplicated. This is the case when using the rollup plugin where each transforms will run the transform method.

https://github.com/bublejs/buble/blob/ecc42f5685810a966bd53b77866028f5322ac7ac/src/program/Program.js#L28

when this is `null`, the helper will be injected, this will always be null when we run transform. Is there a way to reuse it?

https://github.com/bublejs/buble/blob/ecc42f5685810a966bd53b77866028f5322ac7ac/src/program/Program.js#L63-L73

Start in src/index.js around line 95 and src/program/Program.js around lines 28 and 63-73. Trace how each transform creates a Program and injects helpers, then reproduce the Rollup-plugin case; done means repeated transforms no longer emit duplicate helpers such as objectWithoutProperties.

Written by the indexing model from the issue text.