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

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

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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.

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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

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

Surprisingly, a global "nogil" did not work

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extra : transplant_source : GI%ADU%5E%CEId%CET6%90%8D%F8%AB%CCr%3FCy

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extra : transplant_source : %EF%CB8%5D%B9%20%C4%9E%D9%BC8%BD%D9w4%AA%ABk%0B%EB

libcpp.string supports s[0]= and s.at(0)=

rewrite of dict iteration code at a more C-ish level in order to make the actual code C compile time dependent

--HG--
extra : transplant_source : R%BCF%17%5D%7E%FA%B3%C4q%D2%28%CF%0Di%8D%FC%0B%E4%7B