adding support for get-else, get-some and missing? as built-in preds

Not sure if this is the best approach as it required passing along `context` to `-call-fn` in both `filter-by-pred` and `bind-by-fn` to allow resolving `$` to a source. This will also be necessary for `get-some`, `missing?` and similar though so I am going to run with it for now.