Custom warning suppression

[David Feuer]

Currently, the only way to suppress custom warnings and deprecations is
with -fno-warn-warnings-deprecations, which is a rather large hammer. I see
two ways we can improve this, and I propose that we should do both.

1. Per-binding suppression

Add -fno-warn-binding, -fno-deprecate-binding, -fwarn-binding options, and
-fdeprecate-binding options. These would take the (optionally qualified)
name of a binding and control warnings tied to it. So if you invoked
-fno-warn-binding "sillyFunction", then GHC would not warn you about the
silliness of anything named sillyFunction. -fno-warn-binding
"Data.Silly.sillyFunction" would limit the suppression to the silly
function in Data.Silly. -fno-deprecate-binding would refrain from emitting
deprecation warnings for the binding in question. -fno-deprecate-binding
would presumably imply no-warn-binding, since someone who doesn't care that
a function is going to be removed probably also doesn't care what else is
wrong with it.

2. Named warning classes

I'd like to add an optional "warning class" to the WARNING pragma,
preceding the warning description. This would be a short string indicating
what sort of warning is involved. This would be totally free-form, but the
documentation would suggest a few conventional options such as "partial"
and "slow". Then whole warning classes could be controlled with
-fno-warn-class and -first-class.
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