Merge branch 'ab/streaming-simplify'

Code clean-up.

* ab/streaming-simplify:
  streaming.c: move {open,close,read} from vtable to "struct git_istream"
  streaming.c: stop passing around "object_info *" to open()
  streaming.c: remove {open,close,read}_method_decl() macros
  streaming.c: remove enum/function/vtbl indirection
  streaming.c: avoid forward declarations