fix(serde): error instead of returning null for numbers out of range

[dsherret]

Closes #85

Problem

A JSON number too large for an f64 silently deserialized to null:

let j: serde_json::Value = parse_to_serde_value("{\"amount\": 1e400}", &Default::default()).unwrap();
assert_eq!(j.to_string(), "{\"amount\":null}"); // passed

str::parse::<f64> resolves to f64::INFINITY instead of erroring, and serde_json's Value visitor routes non-finite floats through Number::from_f64, which returns None and produces Value::Null. Deserializing to a plain f64 yielded inf.

Fix

visit_number now returns a ParseError when the number can't be represented, so { "amount": 1e400 } errors with Number is out of range on line 1 column 13. This matches serde_json::from_str, which errors with number out of range for the same input.

While in there, the hexadecimal branch had the same class of bug — it fell back to visiting the raw literal as a string when it didn't fit in an i64. It now:

• errors for out of range values (ex. 0xFFFFFFFFFFFFFFFFFF), instead of deserializing as "0xFFFFFFFFFFFFFFFFFF"
• parses via i128 so values that fit in a u64 (ex. 0x8000000000000000) and -0x8000000000000000 (exactly i64::MIN) deserialize as numbers, instead of as strings

The trailing visitor.visit_str(raw) fallback for unparseable decimals is gone since it was unreachable — every number token the scanner produces is valid Rust float syntax, so parse::<f64> only ever fails to give a usable value by overflowing.

Behaviour change

Input that previously deserialized (as null, inf, or a string) now errors, including when the value's field is ignored by the target type — serde_json errors there too.

Note this leaves the non-serde conversions diverging: impl From<ast::Value> for serde_json::Value and CstNumberLit::to_serde_value still turn 1e400 into Value::String("1e400"), and their signatures (From / -> Option<_>) can't surface a parse error. Happy to follow up if you'd like those to return None or be documented.

Tests

Added it_should_error_when_number_is_out_of_f64_range and it_should_error_when_hexadecimal_number_is_out_of_range, covering positive/negative overflow, error positions, the largest finite f64, underflow to ±0.0, out-of-range values in ignored fields, and the hex cases above.