* feat: Remove IE11 transpilation and target ES2015-ish

What we'll be targeting from now on is approximately:

```
last 3 ios_saf versions
last 3 and_chr major versions
last 3 samsung major versions
last 3 opera versions
last 3 safari versions
last 10 chrome major versions
last 8 firefox major versions
last 3 edge versions
not ie
not op_mini
not and_uc
```

* Remove transform-function-expressions babel transform

* Update Rollup config

* Update eslint config to allow ES6 static methods and functions

* Remove IE11 support from @urql/exchange-persisted-fetch

* Add changeset

* Deduplicate dependencies

* Fix cjs-check-plugin

* Upgrade @babel/core and terser

* Permit for-of

* Simplif core code where for-of is needed

* Replace dictionaries with Set where for-of may be used

* Replace several dictionaries with maps

* Prevent empty object allocation if possible

* Simplify ast helpers in graphcache

* Tweak ownership and gc timings

* Switch schema checks to Map rather than dictionary

* Upgrade to wonka@^6.0.0

* Remove invalid import from retryExchange

* Upgrade Rollup/Babel and disable Buble's for-of transform

* Rewrite makeFetchURL with URL/URLSearchParams

As these are built-ins in Node 10+ and in all evergreen browsers we
support, we can just use `new URL`

* Remove Buble entirely

* Fix faulty type in retryExchange

* Update new tests for updated data structures