Rename method fn value() on NodeMut<…> to value_mut(), re-introducing the former as an immutable getter

[regexident]

Currently NodeMut<'a T> only exposes …

pub fn value(&mut self) -> &mut T;

… but not an equivalent immutable …

pub fn value(&self) -> &T;

This PR renames the existing name(…) to name_mut(…) and re-introduces name(…) as immutable getter.

(This is a breaking change.)

While one could assume that anyone using NodeMut will also have mutable access to it I stumbled on situations where this wasn't the case (e.g. due to wanting to abstract over NodeRef/NodeMut for mere reading) and the lack of a non-mutating node() getter made the code rather cumbersome to deal with as a consequence.

The same goes for the lack of fn children(&self) -> Children<'a, T> on NodeMut for what it's worth.

[adamreichold]

I wonder if we can solve the same problem easier by letting code turn NodeMut into NodeRef directly. Something like

fn as_ref(&mut self) -> NodeRef<'_, T>;

i.e. a reborrowing operation. (Note that the lifetime of the NodeRef is shorter than that of the original NodeMut which I think is necessary for soundness.)

Otherwise, we'll just end up duplicating all of the API surface adjusted for mutability.

[regexident]

That should work too, yeah!