Template Haskell: support other FFI calling conventions

In the Template Haskell syntax representations, an FFI calling convention can currently only be CCall or StdCall. I'd like to generate code with GHCJS JavaScript imports, which use a "javascript" calling convention. The FFI section of the Report mentions other potential conventions, and specifically says that implementations can create new ones as appropriate, so perhaps a new constructor could be added which takes an arbitrary string? Something like:

data Callconv = CCall | StdCall | Callconv String
    deriving( Show, Eq, Data, Typeable )

Trac metadata

Trac field	Value
Version	7.8.3
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Template Haskell
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