Refactor input processors to make them more straightforward and performant

[Shute052]

• Replaced processor traits and their impls with enums, enabling static dispatch and achieving a five-fold increase in speed.
• Replaced AxisInverted with AxisProcessor::Inverted.
• Replaced AxisSensitivity with AxisProcessor::Sensitivity(f32).
• Removed input_processing/single_axis/modifier.rs.
• Merged input_processing/dual_axis/modifier.rs into mod.rs.
• Removed input_processing/*/pipeline.rs.
• Renamed process methods of old processors for clarity.
• Renamed old processor traits to Custom*AxisProcessor for defining custom processors.
• Implemented Into<*AxisProcessor> for old processors.
• Added*AxisProcessor::None to eliminate an unnecessary layer of Option.