help keeping track of variable ids

Sometimes you have a "name" for a variable when you create it, and you want to easily go between the "name" and the variable's `TypedID`. Examples:
* https://github.com/borglab/SwiftFusion/pull/108 -> `ChordalInitialization.swift` -> `solveOrientationGraph`: This takes a factor graph `g` as input and constructs a corresponding "unconstrained chordal graph" that has one variable per variable in `g`. (The variables in the "unconstrained chordal graph" have a different type from the variables in `g`). Then it solves the "unconstrained chordal graph" and finally it reads the solution and uses it to construct good initial values for variables in `g`.
* You read a factor graph from a file on disk, and the variables have string identifiers. You want to solve the graph and then write the solution to disk, using the string variable identifiers.

Having a convenient pattern/utility/something for doing this would be convenient.