fix: avoid panic in duration parser on out-of-range values

[ChrisJr404]

While parsing untrusted input I hit a panic in iso8601::duration().

Any duration whose numeric component is larger than u32::MAX crashes the
parser:

iso8601::duration("P99999999999Y"); // panics

thread 'main' panicked at src/parsers.rs:40:
Invalid string, expected ASCII representation of a number: ParseIntError { kind: PosOverflow }

Cause

Every duration component is read by take_digits, which collects a run of
ASCII digits and then converts it with .parse().expect(...):

let res = s
    .parse()
    .expect("Invalid string, expected ASCII representation of a number");

take_digits accepts an unbounded number of digits, so a value that does not
fit into a u32 makes the conversion return Err and the .expect() panics.
Because the panic is a failed Result::expect (not an arithmetic overflow), it
fires in both debug and release builds.

Only the duration grammar uses the unbounded take_digits; the date, time and
datetime parsers use the fixed-width take_n_digits helper (max 4 digits), so
they are not affected. That is also why the existing parse_dates fuzz target
never surfaced this — it exercises date/time/datetime but not duration.

Fix

take_digits now returns a normal parser error when the digits do not fit into
a u32, so the parse fails gracefully instead of panicking. Values up to
u32::MAX still parse unchanged. Added a regression test covering each
duration component and confirming u32::MAX still parses.

cargo test, cargo fmt --check and cargo clippy all pass.