key: implement ROOTUnmarshaler
decoder: use bytes.Buffer as underlying storage.

decoder_test: first import.
attxxx: use NewDecoder + checkByteCount.
basket: use NewDecoder
decoder: introduce NewDecoder. use bytes.Buffer as underlying storage. introduce checkByteCount, Clone, Pos and Len
consts: add constants for streamers
file: use NewDecoder
file: first stab at readStreamerInfo
file_test: proper Key<->Basket handling
named: use NewDecoder + checkByteCount
ifaces: fixup ROOT(Un)Marshaler interfaces to use bytes.Buffer
dir: use NewDecoder
tree: implement Object interface (via named)
tree: use NewDecoder. first stab at reading []Branch and []Leaf
key: use NewDecoder. first stab at proper Basket impl.
tree_test: cosmetics
branch: first import
objarray: first import
objarray: typos