Added locked_axes() and set_locked_axes() methods for (some) joints

[Aztro-dev]

Implemented locked_axes(&self) for all of these structs:

• FixedJoint
• PrismaticJoint
• RevoluteJoint
• RopeJoint
• SphericalJoint

locked_axes(&self) returns all of the axes that are currently locked by the joint.

Implemented set_locked_axes(&mut self, axes: JointAxesMask) for all of these structs:

• PrismaticJoint
• RevoluteJoint
• RopeJoint
• SphericalJoint

set_locked_axes(&mut self, axes: JointAxesMask) allows you to lock axes that aren't already locked by the default axes.

implemented lock_axes(&mut self, axes: JointAxesMask) for all of these structs:

• PrismaticJointBuilder
• RevoluteJointBuilder
• RopeJointBuilder
• SphericalJointBuilder

lock_axes(&mut self, axes: JointAxesMask) works exactly the same as set_locked_axes(), but is only implemented for Joint builders.

Made JointAxesMask and JointAxis publicly available, rather than having to import them through rapier.

[ThierryBerger]

Have you considered getters ?

    pub fn get(&self) -> &GenericJoint {
        &self.data
    }

    pub fn get_mut(&mut self) -> &mut GenericJoint {
        &mut self.data
    }

or deref / derefmut derive ?

[ThierryBerger]

I realized some joints data are pub (SpringJoint, ImpulseJoint), some others are not (those you modified: PrismaticJoint...).

I'm curious why there is a visibility difference there @sebcrozet ?