Add `extend` module to implement conversion from a narrower to a wider
floating-point type.
This implementation is only intended to support *widening* operations.
Module to convert a *narrower* floating-point will be added in the future.
All tests are moved to a separate crate in this repository to enable features by
default. Additionally the test generation is moved to a seprate build script and
simplified to reduce the amount of boilerplate needed per test.
Overall this should still be testing everything, just in a different location!
