Optimize iterators a bit

[lahma]

• use same type for all iterators which has faster shortcuts
• add fast path for array get length
• small cleanup

Jint.Benchmark.ForBencmark

Diff	Method	Mean	Error	Allocated
Old	ForOfVar	164.7 μs	0.35 μs	330.99 KB
New		107.7 μs (-35%)	0.22 μs	119.95 KB (-64%)
Old	ForOfLet	177.4 μs	0.64 μs	635.59 KB
New		139.8 μs (-21%)	0.30 μs	424.55 KB (-33%)
Old	ForOfConst	178.6 μs	0.45 μs	635.59 KB
New		144.3 μs (-19%)	0.38 μs	424.55 KB (-33%)
Old	ForVar	159.9 μs	0.67 μs	3.66 KB
New		152.4 μs (-5%)	1.62 μs	3.66 KB (0%)
Old	ForLet	210.1 μs	0.44 μs	191.42 KB
New		211.8 μs (+1%)	3.48 μs	191.42 KB (0%)