A user-defined conversion function is an instance method that accepts
an empty tuple and returns a value of the type we're converting to,
has the [conversion] attribute, and is named __conversion. The last of
these restrictions is a temporary hack to work around our inability to
perform a lookup across all extensions for "every function with the
conversion attribute", and shouldn't last too long.
As in C++, we only get one user-defined conversion function. Unlike in
C++, a constructor is not (and cannot) be a conversion function.
Introduce NSString <-> String conversion functions, but leave the
runtime implementations as stubs for Dave to fill in.
Swift SVN r1921
