improve performance building large documents with XML::Builder

[flavorjones]

some benchmark numbers:

                                                    user     system      total        real
nokogiri: 1000 docs, 10 stories, to string      5.180000   0.470000   5.650000 (  5.663716)
nokogiri: 100 docs, 100 stories, to string      5.500000   0.480000   5.980000 (  5.984575)
nokogiri: 10 docs, 1000 stories, to string      7.900000   0.490000   8.390000 (  8.409311)
nokogiri: 1 docs, 10000 stories, to string     23.530000   0.500000  24.030000 ( 24.247925)

[flavorjones]

removing O(n) penalty in node new/unlink/reparent by replace xmlXPathNodeSet with a hash. closed by f34f3bd.

[flavorjones]

new benchmarks:

                                                        user     system      total        real
nokogiri: 1000 docs, 10 stories, to string      4.980000   0.530000   5.510000 (  5.546052)
nokogiri: 100 docs, 100 stories, to string      5.360000   0.520000   5.880000 (  5.900826)
nokogiri: 10 docs, 1000 stories, to string      6.360000   0.560000   6.920000 (  6.973381)
nokogiri: 1 docs, 10000 stories, to string      6.270000   0.500000   6.770000 (  6.783409)

[flavorjones]

and just for posterity, here is the same documents generated by Builder::XmlMarkup (the Rails default builder):

builder: 1000 docs, 10 stories, to string      13.570000   1.300000  14.870000 ( 14.887572)
builder: 100 docs, 100 stories, to string      13.470000   1.270000  14.740000 ( 14.748797)
builder: 10 docs, 1000 stories, to string      13.210000   1.310000  14.520000 ( 14.544887)
builder: 1 docs, 10000 stories, to string      13.370000   1.280000  14.650000 ( 14.889584)