Extend ship data type by hit times. (#139)
Determine sinking times by hit times in the Ship type. (fixes #139)

Additionally, make the rendering code a little nicer by using
`Time` as a maximum monoid.
Display ships that are not sunk on top of sunk ones.
Replace "unsafe" function `shipAt` by `shipsAt`.
Merge branch 'master' of https://github.com/zrho/afp