Remove check for default base constructors when declaring a constructor.

This seems to have been a mis-understanding on how C++ works/is wrapped.
Test for non-default constructor in bases.
Merge branch '0.21.x'
add some missing CPython 2.x declarations
explain how to use language='c++' before Cython 0.21.1 (patch by Daniele Nicolodi)
add missing std::string to str convertion helper
factor out redundancy in std::string -> Python object helper code
merge 0.21.x branch into master
use simple memcpy() instead of explicit C array copying
extend C array coercion tests
improve test
allow list literals with a constant multiplier to be assigned to C arrays, extend array assignment tests
avoid duplication of "stringsource" entries in filename table

--HG--
extra : amend_source : c0a1e78dbfd443585a80bd646b763617eeb6b7c5
fix a case where C array coercion helper code could be duplicated in the C file
speed up cythonized hexiom2 benchmark a little by using C int arrays instead of fixed size Python lists