Fix performance bug in std.find implementation

It turns out std::string::find is way slower than strstr... Thanks to g
webb for spotting this!
Fix bug when spec contains a tuple of the same type

For instance #pythran export foo((int, int))

The trick was to use core::make_tuple instead of relying on hardcoding std::tuple

Add test cases from g webb that exhibits this behavior
Merge pull request #158 from serge-sans-paille/small-fixes

Small fixes