add in method

[stev47]

This change makes in unroll the loop (compared to Base.in) and is
significantly faster.

[coveralls]

Coverage increased (+0.007%) to 81.688% when pulling 50d62fe on stev47:add-in into 41728ad on JuliaArrays:master.

[c42f]

Looks good to me, the generated code is much better for numeric types. It's much faster for short vectors and still a lot better for static arrays of length 16:

julia> f(x) = x in (1,3,2,5,6,7,8,10,11,12,13,70,80,81,82,16)
f (generic function with 1 method)

julia> g(x) = x in SVector(1,3,2,5,6,7,8,10,11,12,13,70,80,81,82,16)
g (generic function with 1 method)

# Wrap in mapreduce to prevent optimizer from removing `f` entirely (??)
julia> @btime mapreduce(f, |, $(rand(Int, 1000)))
  6.086 μs (0 allocations: 0 bytes)
false

julia> @btime mapreduce(g, |, $(rand(Int, 1000)))
  1.276 μs (0 allocations: 0 bytes)
false

Side note: here's an entertaining optimization opportunity exposed by this PR:

julia> g(x) = x in SVector((1,2,3,4))
g (generic function with 1 method)

julia> @code_native debuginfo=:none g(1)
	.text
	addq	$-1, %rdi
	cmpq	$4, %rdi
	setb	%al
	retq
	nopl	(%rax)