Avoid Hash allocations in combine
Avoid Hash allocations in combine

Before

                          user     system      total        real
    combine 50 items with 3 levels of nesting  0.480000   0.000000   0.480000 (  0.479131)
    combine 100 items with 3 levels of nesting  3.560000   0.020000   3.580000 (  3.570935)
    combine 200 items with 3 levels of nesting 28.130000   0.070000  28.200000 ( 28.192266)

After

                          user     system      total        real
    combine 50 items with 3 levels of nesting  0.390000   0.000000   0.390000 (  0.393347)
    combine 100 items with 3 levels of nesting  3.040000   0.000000   3.040000 (  3.036016)
    combine 200 items with 3 levels of nesting 25.630000   0.000000  25.630000 ( 25.615616)

solnic/transproc