Object Pool: remove space in "in- place"

Line-break after "in-" made a space in "in- place". Fixed.
Revise Singleton.
Revise State and Double Buffer.
Fix #210.
Fix #211.
Merge pull request #212 from Kernald/patch-1

Object Pool: remove space in "in- place"
Fix #213.
Fix #216.
Fix #217.
merge with upstream