ES6 Number APIs

[lahma]

• quite a lot of work as missing Dtoa functionality now ported to finally fix the issues with toPrecision (ToPrecision() not being evaluated as expected #459)
• the Dtoa code is intentionally C style syntax for it to be easier to diff with v8 code base, so maybe we can be less strict about stylistic issues
• converted builder to be simpler to follow v8 code base and moved formatting to prototype
• unified string builder pool to work like other pools, sharing dtoa builder instance via engine, like pools
• new tests included & passing

[sebastienros]

I wonder if we can do better with the SB pool. Now we need to pass (and have) the engine for most type conversions because of it. First question, do we really need a pool of instance, or just one reusable instance, as in "are there recursive calls" that will issue multiple SBs?

Should/could the SBs be shared by all Jint engines, using a static pool? I assume it matters in a server scenario only where concurrency is more an issue, but at the same time it could also be an improvement as all threads could reuse the same SB overtime. Another option would be to have a static pool of SB pools, to reduce concurrency by reusing the pools across engine instances.

[lahma]

I've added ConcurrentObjectPool which is the same pool with proper Interlocked support, so now StringBuilderPool is a global pool and API changes including new engine parameter reverted. I think that finding optimal non-concurrent pool might a separate task.