implementation of PEP 380 (yield from)

optimised tuple/list/iterable unpacking

merging _dict_iter_rewrite branch after discussion on cython-devel list

Add "nogil" to the stdc++ pxd files

DOC - partial rewrite of fused types doc

use normal loop for larger numbers of items in tuple unpacking instead of unrolled loop to reduce code overhead

Ignore byte compiled elisp modules in the repository.

* When cython.git is included as a submodule in an
  Emacs configuration to load cython-mode.el and is
  byte-compiled, it generates an .elc file which causes
  the submodule to appear dirty unnecessarily in the parent
  git repository. Adding this rule will avoid dirty submodules.
Signed-off-by: Yesudeep Mangalapilly <yesudeep@google.com>

include fast list/tuple iteration in dict iteration optimisation to prevent performance regressions for the too-optimistic optimisation case

to help in breaking potential reference cycles, clear references in generator objects immediately on termination instead of waiting for them to be cleaned up

--HG--
extra : transplant_source : %28%90%91h.%EE%94%1AE%F5%A9%DA33J%D0%60h%F5t

After conversation with Mark F today. Change from pointers to scalars
for clarity.  Add quickstart section.  Explain case where the same fused
type in an argument list must be specialized to same type.

--HG--
extra : transplant_source : %9F%0B%FF%0F%16%0C%25%B9%FA7%D8%25%9EDpJSE%5E%BB

--HG--
extra : transplant_source : %7D%F5%F9%9Eq%8E%EA%1B%27%88Q%9E%85%C25qE%A2%F8%C3

--HG--
extra : transplant_source : %2B%AF%ADGa7%08O%FCC%40%08%AAI%DF%FE%22F%0C0

--HG--
extra : transplant_source : %AE%FF9%0ALO%CE%5E%C7%93%FA%B1d%E73%E8%CA%BE%DC%C8

use optimised dict iteration also for untyped objects when the known iterkeys/values/items() methods are used

--HG--
extra : transplant_source : %FAOM%F4%BF%9E%B68%F0%E6%A2%C8P4sz%F4%B7%DBG

--HG--
extra : transplant_source : %E3%97%5E9%1A%97%9BR%EC%01%FD%A0%B7-%81%C5G%AD%DA%DC

--HG--
extra : transplant_source : %B2Q%A0%A7c%3B%EE%40%93%08%E1%C9-%83%88%BA%ACU%7D%8D

--HG--
extra : transplant_source : %D5V%9A%D2%DE%A7%A8%20%21%05%FFp%DEk%C4f%3Ct%0DT

Surprisingly, a global "nogil" did not work

--HG--
extra : transplant_source : %21G%91%9Cy%EB%90%1Al%06%13%3B%C0%CB%0D%10%068%DC%E9

--HG--
extra : transplant_source : %E8%28%A4n%05%B9Q%7E%BBXuXn%21f%05c%60%5B%AB

--HG--
extra : transplant_source : %04E%21%DB%F8%D4%C0%FD%C7%11%5E%F1%28%C6%26R%E9%06%CE%FD