Make ChType a member of IntTy ITChar.

This enables back end specific character encoding and unifies some operations.
Note that there is no Ord instance, so
```
'a' + 1
```
will not work. However
```
prim__addChar 'a' (prim__truncBigInt_Char 1)
```
is now possible and client libraries may feel free to put it to good use (e.g. checking for character ranges).
Removed FChar in favour of FArith (ATInt ITChar)
Merge pull request #383 from JanBessai/ITChar

Make Char part of ITType