Fix panic when setting RTreeParams MIN_SIZE to 1

[urschrei]

rstar's capacity-calculation logic would attempt to set the underlying vec's capacity to 2^64 -1 as a result, causing a panic because this exceeds the max value of isize on 64-bit platforms. This fix adds a check for MIN_SIZE = 1 when calculating capacity.

The fix currently sets the max depth to 1 – should it be higher?

Fixes #92

• [x] I agree to follow the project's code of conduct.

• [x] I added an entry to rstar/CHANGELOG.md if knowledge of this change could be valuable to users.

[urschrei]

Blocked on https://github.com/georust/rstar/issues/94

[urschrei]

bors r=rmanoka

[bors[bot]]

Build succeeded:

• ci result