Add array fusion versions of map, filter and foldl

Add array fusion versions of map, filter and foldl

This patch adds fusable map, filter and foldl, using the array fusion
code for unlifted, flat arrays, from the Data Parallel Haskell branch,
after kind help from Roman Leshchinskiy,

Pipelines of maps, filters and folds should now need to walk the
bytestring once only, and intermediate bytestrings won't be constructed.