Make the maximum tuple size accesible

[fons]

In relation with the implementation of a Haskell-embedded DSL, I'm using Template Haskell to automatically generate instances for tuples.

I'd like to use tuples as big as GHC allows me to. However, the upper size limit (62 as of GHC 6.8.2) is not accessible by the end user. That forces me to hardcode the size, which, as you can imagine, is not a good practice since the value can change in future versions of the compiler.

Thus, I propose to include a constant (e.g. maxTupleSize) somewhere under GHC.* (maybe GHC.Exts) for that purpose.

[igloo]

I've made it available in the HEAD:

Prelude> GHC.Exts.maxTupleSize
62

We should probably put it somewhere more sensible, e.g. Data.Tuple, at some point.