Fix ceylon/ceylon-compiler#933 by adding type argument to return type of Array.withLeading() and Array.withTrailing()
Implemented toplevel `combine()`
Deleted Java implementation of toplevel `combine()`
Merge branch 'master' of https://github.com/ceylon/ceylon.language into
533

Conflicts:
	runtime/ceylon/language/combine_.java
	src/ceylon/language/combine.ceylon