Callbacks allocate due to type instability

These calls to `nlp_data` are not type stable, so every callback allocates. Can we fix this? One option is something like #514

https://github.com/jump-dev/Ipopt.jl/blob/077843b3b77b5946011c7656c267fc272a0427f0/ext/IpoptMathOptInterfaceExt/MOI_wrapper.jl#L1033

Start in ext/IpoptMathOptInterfaceExt/MOI_wrapper.jl around line 1033, where the issue identifies the type-unstable nlp_data calls. Compare the proposed approach with issue #514, then verify that the callback path is type stable and no longer allocates during callbacks.

Written by the indexing model from the issue text.