This commit adds a `grain/grain.js` module, which contains a type and
logic for representing Grain balances with 18 digits of precision. We
use the native BigInt type (and add the necessary babel plugin to
support it).
Unfortunately, Flow does not yet support BigInts (see
[facebook/flow#6639]). To hack around this, we lie to Flow, claiming
that BigInts are numbers, and we expect/suppress the flow errors
whenever we actually instantiate one. For example:
```js
// $ExpectFlowError
const myBigInt = 5n;
```
We can use the BigInt operators like `+`, `-`, `>` without flow errors,
since these actually exist on numbers too. However, flow will fail to
detect improper combinations of regular numbers and BigInts:
```js
// $ExpectFlowError
const x = 5n;
const y = x + 5;
// Uncaught TypeError: Cannot mix BigInt and other types
```
Since any improper mixing will result in a runtime error, these issues
will be easy to detect via unit tests.
In addition to adding the basic Grain type, I exported a `format`
function which will display Grain balances in a human readable way.
It supports arbitrary decimal precision, groups large amounts with comma
separators, handles negative numbers, and adds a suffix string.
The format method is thoroughly documented and tested. Thanks to @Beanow
for valuable feedback on its implementation.
Test plan: See included unit tests. `yarn test` passes.
[facebook/flow#6639]: https://github.com/facebook/flow/issues/6639
I moved `config/babel.js` to `.babelrc.js` because it seemed like babel
7 really wanted that. I also blew away our (complicated, copied from
create-react-app) config and replaced it with a much, much simpler one.
Test plan: `yarn test` passes, `yarn start` still serves a working
server, and `scripts/build_static_site.sh` still produces a working
site.
Possibly we lost some nice features re: React debugging; if so I'll add
them back as I miss them.
Dandelion Mané