replace 'const_xyz' work-arounds in standard .pxd files by real 'const' declarations

--HG--
extra : transplant_source : H%91%CF%08t%B1%908%AE%26%81%1B%F9%2C%9A%3Fh%ECWK
update 'const' section in string handling tutorial to reflect the new 'const' language support

--HG--
extra : transplant_source : U%15%0B%8E%81%02%F2kE%AA%07u%EF%82%3D14%F1C%86
fix cimport in libcpp.string

--HG--
extra : transplant_source : %E6%CA%F8%11%E8%81u%B9%95%3D%27%C1%0F%F3O%8A%12%3Cnl
Use OS-dependent directory separator - a / on windows is interpreted by LINK as a command line switch

--HG--
extra : transplant_source : %A8%F23%AF%26%BC%82y1%86S%1Ac%D3%40%089o%DCQ
fix import of pyx modules when '' is in sys.path

If '' is in sys.path and a module is found the package_path
is relative and breaks the build process.

--HG--
extra : transplant_source : 9%EA%CC%A6%3D%1B9R%EF%0DmM%CFZ%18%F3%EC%06%3B%B7
fix cimport in libcpp.string
changelog
merge 0.18 branch back into master