Rename BLTFilter to Filter.

[yara-blue]

https://github.com/RustAudio/rodio/blob/22d90e71d8d635bb777f5c0f9ca95be843b02ff3/src/source/blt.rs#L58

The filter method Billinear transform, is an implementation detail that should not appear in the API, since that limits future change. That alone is not a reason to rename it.

The current name is confusing to new novice users however and if we do a breaking release with porting/update guide that is a good moment to fix this.

Should not impact many users as most will not have specified the complete type of their source.

Still this is very much still open to discussion.