Proposal: modify `Read` instances for `Float` and `Double`

[Iavor Diatchki]

Hello,

I am working on a small GHC extension to support floating point literals in
hexadecimal notation (https://github.com/ghc-proposals/ghc-proposals/pull/37),
which is similar to what's available in other languages.

To support this change, I would like to propose that we modify the `Read`
instances for `Float` and `Double` to parse literals in the new notation.

This may affect existing programs---although it doesn't seem very likely.
Here is an example:

current behavior:

reads "0x10p10" = [(16.0,"p10")]

new behavior:

reads "0x10p10" = [(16384,"")]


What do people think?

-Iavor
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