Support roundtripping of numbers

Slightly related to #679

There are edge cases with Number where we don't roundtrip, e.g.
```
λ encode (decode "3.00e-3" :: Maybe Value)
"3.0e-3"
```
We've also had to add restrictions on parsing Scientific to prevent security issues.

I'm wondering if we should change Number to a type that essentially just stores the original text representation. This way we can properly roundtrip all numeric values, and we can disallow NaN/Infinity per the spec.

Start by examining the Number representation and the Scientific parsing restrictions described in the issue, using the 3.00e-3 example as a reproduction. Determine the intended design before changing anything; done should include correct numeric roundtripping and rejection of NaN and Infinity according to the JSON specification.

Written by the indexing model from the issue text.