Bug 1038038 - Part 1: Reduce ShapeTable::MIN_SIZE_LOG2. r=bhackett, a=lmandel

An entryCount of zero is common, due to initBoundFunction(), so it's worth
choosing a value that doesn't waste too much space in that case.
Bug 1038038 - Part 2: Compute sizeLog2 in ShapeTable::init() better. r=bhackett, a=lmandel

For exapmle, currently if you have an entryCount of 9 you end up with a
capacity of 32, when 16 would be more appropriate.
Bug 1038038 - Part 3: Increase ShapeTable::MIN_ENTRIES. r=bhackett, a=lmandel

A linear search of 15 items is likely to be competitive with a hash table
lookup.