Selective disabling of unused bind warnings

[ajd]

I like to compile with -fwarn-unused-binds to keep my code clean, but I think a couple of extra flags to control this would be useful:

* -fno-warn-unused-fields which would turn off warnings for record field names that were not used. I know you can prefix them with _, but then when you do use them then you have to change the name, etc.

* A {-# USED f, g, h, ... #-} pragma to tell the compiler not to warn about the functions given in the pragma.

I can see how especially the second could be controversial as it introduces an incompatibility, but I think both of these features could be put to good use. Not very high priority, though.