Benchmarks to measure future improvements

[hughfdjackson]

A benchmark suite would allow us to measure the optimisations that happen hereafter.

[hughfdjackson]

As stated, this ticket doesn't encompass what is really needed.

As a contributer, I want to be able to compare the performance of my changes in a representative test suite vs older version of the library, and against mutable version. As a library user, I want to be able to assess the library's performance via the benchmark; and also see what progress is being made.

Acceptance:

• A suite of benchmarks that can be run against the major modern javascript engines; such as those present in: * Chrome/node * Firefox \ IE
• The suites must be easily extended across basic API naming changes, to accomodate the dev in that area
• The suites (or a subset of them) must run over common use-cases in an acceptably short amount of time.
• The suites must be able to be run by interested parties - linked from the readme of this repo/the node package page.
• Each immutable action has a mutable counter-part for comparisons.

[hughfdjackson]

This requirement will be null and void with the inclusion of immutable-hash-trie.