seq: add support for parsing hexadecimal floating point inputs

[jfinkels]

See https://www.gnu.org/software/coreutils/manual/html_node/Floating-point.html#Floating-point

For example,

$ seq -0x.ep-3 -0x.1p-3 -0x.fp-3
-0.109375
-0.117188

[stale[bot]]

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[Qelxiros]

Hello! I started digging into this, and it looks like BigDecimal has some shortcomings. I'd like to use the BigDecimal::from_str_radix() function to parse input (either before or after splitting on p), but that function only allows a radix of 10. I considered some sort of hack to work around this, but at that point I'm just reimplementing what bigdecimal should do. I'm going to look into making a PR to bigdecimal, but failing that/in the meantime, I'd appreciate some guidance here. Thanks!