Factor out code duplication in src/test/bench/core-map.rs

[thestinger]

The benchmarks for LinearMap and TreeMap should just be using the Map trait, but I encountered a strange borrow checking issue:

• diff: https://gist.github.com/4616504
• error: https://gist.github.com/4616785

This seems like a borrowck bug, but I'm not sure. The same issue occurs when trying to work around it with a trait like Construct with a new method.