Provide a way to optimize allocation amounts in `Unique/RigidArray`

[MahdiBM]

When asking for reserving capacity, Swift's Array takes it as "minimum capacity" that it should reserve.
Unique/RigidArray though, take it very literally and only allocate those exact amounts:

import BasicContainers

var uniq = UniqueArray<UInt8>(capacity: 10)
var arr = Array<UInt8>()
arr.reserveCapacity(10)

print(uniq.capacity) /// prints 10
print(arr.capacity) /// prints 16

I think It's likely better to just copy paste the algorithm that Array uses for reserving capacities?
Or there could be new functions / initializers for this purpose.

I just recently moved swift-idna to use Unique/RigidArray instead of Array, and noticed this behavior.

@lorentey WDYT? I could do the PRs if this sounds like a good idea to you.