FR: add `symbol`/`reference` module or similar

Currently there's no cross-platform way of denoting a globally unique value across runtimes. `gleam_javascript` can make JavaScript symbols, while `gleam_erlang` allows for making unique Erlang references. A simple module that allows creating the core concept of a unique value would be nice. The module would expose an external type, `Symbol` or `Reference` or `Unique` or `Special` or whatever the name would be, as well as a `new()` function that creates a new value. Equality between two separately "instantiated" symbols or references would fail. I feel like the language-specific constructs are similar enough to have some sort of shared concept in the standard library.