Adds concat.
Adds head.
Groups functions by general purpose.

--HG--
rename : src/as-array.ls => src/common/as-array.ls
rename : src/sort-by.ls => src/common/sort-by.ls
rename : src/without-first.ls => src/common/without-first.ls
rename : src/maximum-by.ls => src/folds/maximum-by.ls
rename : src/minimum-by.ls => src/folds/minimum-by.ls
rename : src/find-index.ls => src/searching/find-index.ls
rename : src/find-indices.ls => src/searching/find-indices.ls
rename : src/find.ls => src/searching/find.ls
rename : src/intersect-by.ls => src/sets/intersect-by.ls
rename : src/union-by.ls => src/sets/union-by.ls
rename : src/unique-by.ls => src/sets/unique-by.ls
rename : src/group-by.ls => src/transformations/group-by.ls
rename : src/intersperse.ls => src/transformations/intersperse.ls
rename : src/partition.ls => src/transformations/partition.ls