Now we allow objects with `f32` or `f64` string-serialized keys. For example:

```json
{ "1.123": "foo", "2.0": "bar" }
```

The `test_deny_float_key` test has been inverted, and the
`deserialize_integer_key` macros have been renamed to
`deserialize_numeric_key` in order to support `f32` and `f64`.

This handles NaN and both positive and negative infinity.

This ensures consistency with the default `Formatter::write_f32` output
format used elsewhere.

`deserialize_numeric_key!` has been renamed back to
`deserialize_integer_key!`, and a new float-specific
`deserialize_float_key!` macro has been introduced.

This new macro attempts to deserialize float keys by delegating to the
equivalent parent deserializer method.

An `unwrap` has been changed to simply discard the `Result`. We assume
that writing to a `Vec` will never fail because the `std::io::Write`
implementation never creates an I/O error.