Improve getindex performance on ChainedVector with UnitRanges

[quinnj]

Before:

julia> using SentinelArrays, Chairmarks

julia> function f(inputs, range)
           ChainedVector(inputs)[range]
       end
f (generic function with 1 method)

julia> function g(inputs, range)
           reduce(vcat, inputs)[range]
       end
g (generic function with 1 method)

julia> vinputs = [rand(Float32, rand(1024:2048)) for _ = 1:90];

julia> @b f(vinputs, 1:95000)
64.666 μs (9 allocs: 372.125 KiB)

After:

julia> @b f(vinputs, 1:95000)
13.875 μs (9 allocs: 372.125 KiB)

[quinnj]

Fixes #105