Go, Guile, Racket, Scilab: Add throws typemaps for std::string so that
thrown string exception messages can be seen.
Test all language for std::string throws typemaps
longer as of Go 1.5. In Go 1.5 or later user calls to
_swig_makegostring will fail at link time.
Instead, use goout and godirectorin typemaps to allocate strings in Go
code.
Change the Go typemaps support to ignore empty strings, so that we can
define empty strings for regular types so that %apply will override
the definitions for string types.
Fix the gccgo code to wrap SwigCgoCallback around all godirectorin
typemaps.
Add a few newlines after typemap code so that the typemaps don't have
to include them.
