[Improvement] Creep.getActiveBodyparts CPU cost correction

[iamgqr]

The CPU cost of Creep.getActiveBodyparts should be "low" instead of "insignificant". Here's my reason.
Went for the engine and found that the actual codes of Creep.getActiveBodyparts is down below,

function _getActiveBodyparts(body, type) {
    var count = 0;
    for(var i = body.length-1; i>=0; i--) {
        if (body[i].hits <= 0)
            break;
        if (body[i].type === type)
            count++;
    }
    return count;
}

This loop can run up to 50 times in total, which may be a little costy and should not be "insignificant". And also, this result is not cached, which is worth noting in the api.
Spotted this problem after using a lot Creep.getActiveBodyparts and it ended up being the most CPU-costing in screeps-profiler.
I'm not a native English speaker, please excuse my grammar mistakes.