Reduce allocations in SelectorMap (Rule Hash) code

[pradeep90]

• Replace insert_or_update_with with find_mut + insert because the default argument to the first function leads to an allocation every time
• Do the Rule to Declaration conversion in a for loop instead of using map

[emberian]

Probably closedby #1299. I don't understand why you think "Do the Rule to Declaration conversion in a for loop instead of using map" is a good thing, though.

[pradeep90]

@cmr Oh yeah, you're right.

For some strange reason, I thought vec.map() will be more inefficient than for loop / iter().map().
But I see that vec.map() is written in terms of iter().map(f).collect(). Anyway, it is deprecated now.

And like you said, iter().map() will lead to vec using with_capacity instead of possibly allocating one more element each time as in a for loop. I should have stuck to the idiom.

I will change this on Monday. Thanks for the correction.