Rename "key" to "name" according to JSON terminology

Let's stay with the terms used in RFC 4627
Do not modify number strings in parser

We pass the number string to the JsonNumber constructor. So let's be
consistent and not modify the number string in the parser at all.

- case of exponent remains unchanged ('e' vs. 'E')
- redundant '-' is not stripped from a zero
- redundant '+' is not stripped from an exponent (e.g. '1e+5')