Introduce PointerData

[vitvakatu]

Fixes #35
Pointer's size is 16 bytes now, instead of 24. Performance:
before(current master)

test bench_build  ... bench:   1,868,377 ns/iter (+/- 408,680)
test bench_update ... bench:      28,267 ns/iter (+/- 336)
-------
test bench_build  ... bench:   1,065,958 ns/iter (+/- 2,806)
test bench_update ... bench:       4,110 ns/iter (+/- 31)

After:

test bench_build  ... bench:   1,877,622 ns/iter (+/- 305,919)
test bench_update ... bench:      19,316 ns/iter (+/- 110)
-------
test bench_build  ... bench:   1,013,144 ns/iter (+/- 18,468)
test bench_update ... bench:       3,930 ns/iter (+/- 145)

UPD: after PR by @DaseinPhaos performance became slightly better than before.