`seq 1 0x1p-1 2` should be accepted

[Sylvestre Ledru]

with GNU:

1
1.5
2

with our:

seq: invalid hexadecimal argument: '0x1p'
Try 'target/debug/coreutils seq --help' for more information.

[Ben Wiederhake]

Duplicate of #2660. I'll transfer the "good first issue" to there.