SSize type for Foreign.C.Types

Motivation

The ssize_t type is not included as part of the Foreign.C.Types module. The type is used as a return value of various system calls, such as read(2). The type is part of the POSIX standard, hence it is present on the platforms supported by GHC. Having the type available for FFI mappings is useful.

Proposal

Well, the idea is rather straightforward: the base library should be enriched with the CSSize type that directly maps to the C type ssize_t on the respective platform/OS.

Possible Implementation?

In ghc/libraries/base/src/Foreign/C/Types.hs

-- | Haskell type representing the C @ssize_t@ type.
-- /(The concrete types of "Foreign.C.Types#platform" are platform-specific.)/
INTEGRAL_TYPE(CSSize,"ssize_t",HTYPE_SSIZE_T)

The HTYPE_SSIZE_T appears to already exist as a C define in the HsBaseConfig.