Add map_reduce

[jasone]

map and reduce compose, but if separate functions they communicate via a composite data structure that would require compiler heroics to see through. Since these operations are commonly used together, provide a map_reduce function for all container types which implement map and reduce.

[jasone]

The API should probably look like

val map_reduce 'a 'b >m >r: map:(a >m-> b) -> reduce:(b -> b >r-> b) -> t a >{m|r}-> option b
val map_reduce_hlt 'a 'b >m >r: map:(a >m-> b) -> reduce:(b -> b >r-> b) -> t a >{m|r}-> b