Add an experimental RotatedNoise.

This commit adds an experimental type for rotating noise sources,
which is part of #5. It also fixes a few compiler warnings.

Signed-off-by: Atheriel <atheriel@gmail.com>
Improve FunctionNoise and remove the outdated mutability/closure docs.

Signed-off-by: Atheriel <atheriel@gmail.com>
Remove remaining tab characters in favour of spaces.

Signed-off-by: Atheriel <atheriel@gmail.com>
Add a general-purpose ModifierNoise.

The implementation follows the template established by FunctionNoise.
This commit contributes to #5.

Signed-off-by: Atheriel <atheriel@gmail.com>