Explicitly static cast to long double

[chiphogg]

This fixes a very verbose warning that I noticed when installing the
library in a Visual Studio project on Windows. And it just seems like a
clearly good change for its own sake, anyway.

[geoffviola]

It makes sense to add the cast to be explicit.

In a future PR, we should compile with /W4 for Windows. There might be additional warnings that we're missing out on especially when we compile the tests. For example, there are a few in just the single file header: godbolt.

[chiphogg]

Kind of amazed that pulling and merging correctly applied this change to the file's new home!

[chiphogg]

It makes sense to add the cast to be explicit.

In a future PR, we should compile with /W4 for Windows. There might be additional warnings that we're missing out on especially when we compile the tests. For example, there are a few in just the single file header: godbolt.

Good suggestion! I put up a draft PR (#269). We'll see how the build goes. At any rate, that'll help prevent us from forgetting to do this.