move implicit_cast to core?

[HDembinski]

The boost.conversion library contains implicit_cast, which has a simple implementation, no external dependencies and seems useful to have in boost.core. It would also be nice to document the cast better, for me https://stackoverflow.com/questions/868306/what-is-the-difference-between-static-cast-and-implicit-cast was useful to read.

[anshu-khare-design]

I have worked on this.

[glenfe]

@HDembinski I don't think we want to do this. Unlike say, use_default, this doesn't exist on public interfaces and is trivial enough to live in detail namespaces if a dependency on Conversion is not desirable. No dependency cycles are broken or would be avoided by relocating this facility.