uucore: parse_time: avoid expensive with_scale(9) for huge inputs

[loskutov]

The conversion routine used to parse the time into a BigDecimal and normalize it into nanoseconds; however, for an input like 1e6666666666668320 it would require allocating a number with 6666666666668329 zeroes, which is obviously suboptimal (and redundant, as Duration uses u64 for seconds).

Fixes #8634.

[codspeed-hq]

CodSpeed Performance Report

Merging #8740 will not alter performance

_{Comparing loskutov:patch-1 (3d727ca) with main (7dbeb8f)}

Summary

✅ 55 untouched
⏩ 1 skipped¹

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

Skip an intermittent issue tests/misc/tee (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/timeout/timeout (passes in this run but fails in the 'main' branch)
Congrats! The gnu test tests/tail/overlay-headers is no longer failing!

[cakebaker]

Thanks for your PR!