readHexRational missing from Numeric

The HexFloatLiterals extension was merged as result of https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0004-hexFloats.rst

As part of this proposal showHFloat was added to Numeric to output floating points in hexFloat format, unfortunately, no one seems to have remembered to add readHexRational or a similar function to parse hexFloats to Numeric. This leads to the awkward scenario that I can easily produce hexFloat values, but have no way to parse them from String/Text data...

All the code already exists in compiler/util/Utils.hs, someone just needs to cherry pick the implementation add add it to Numeric in base.

Trac metadata

Trac field	Value
Version	8.6.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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