drop is_streamable

there is not a sane and robust way in sight to properly implement it,
so it is better to drop it entirely instead of to provide an half-baked,
incomplete and buggy implementation.

--HG--
extra : amend_source : b84147df3fc3e868cc400b0ed403a096cfe7c3e9
make use of shortcuts.
improve the generic decode().

Catch StopIteration and convert to EOSError: it has no sense otherwise.
Remove the special bootstrap fetch case.
leverage the shortcuts.
require explicit formats for safety.

Avoid works-by-coincidence (or luck). Explicit is better than implicit.