Unify flag descriptions to generate both docs and code
We want to specify all of GHC's flags in one place, and from there
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Generate the flag reference section of the documentation
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Generate the code to parse the command-line
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Allow flags to be deprecated, generating a warning
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Have an option to list all the flags that GHC supports (see #1226 (closed))
for use in command-line completion
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Generate the man page
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Completion in GHCi
This ticket replaces part of #1226 (closed).