Longdouble

This crate provides the type alias c_longdouble for FFI interoperability with C's long double. It does not provide any new types.

The exact type that c_longdouble aliases to is based on https://hackmd.io/@8W5l8q6-Qyyn_vKh2-L0RQ/rJpZsmcdh. Currently the only bindings available are for targets where long double is f64. Once the f128 and f116 RFC becomes usable, platforms that use f128 will be supported. If the additional float types RFC lands, bindings for f80 will also be supported.