[C++-sig] Boost.Python v2: implicit conversion support

[David Abrahams]

Support for C++ implicit conversions has now been added to the v2
codebase. Please see libs/python/test/implicit.* for examples.

The facility looks like:

    boost::python::implicitly_convertible<SourceType,DestType>();

which declares that SourceType is implicitly convertible to DestType.
This declaration allows a Python object which can be converted to
SourceType to be passed to a wrapped C++ function in place of a DestType
or DestType const& parameter.

-Dave

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                  David Abrahams
      C++ Booster (http://www.boost.org)               O__  ==
      Pythonista (http://www.python.org)              c/ /'_ ==
  resume: http://users.rcn.com/abrahams/resume.html  (*) \(*) ==
          email: david.abrahams at rcn.com
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