`getproperty` is type-unstable

Just chased a type-instability in some code back to the `getproperty` method for `LDLFactorization`s [here](https://github.com/JuliaSmoothOptimizers/LDLFactorizations.jl/blob/cf1532ffb0a74b916208e085f42f925ae10bbc15/src/LDLFactorizations.jl#L849). Using `getfield` instead is a workaround, but it would be nice if the dot-property syntax was type-stable, or if there were type-stable "getter" functions for the factorization's fields.