Rename constructors of collections

[jiribenes]

Follow-up from #477

In array, ref, bytes, and other collections:

1. Rename unsafeFresh to allocate and clearly document that it is unsafe
   • perhaps we should decide if we want to have collection::unsafeFunction or collection::unsafe::function in general for functions acting on the collections that are generally unsafe
2. Rename fresh (formerly fill) to $collection_name or $Collection_name, for example array or Array
   • we also discussed using new instead, but that's: a) ambiguous (as now we need the array:: qualifier to determine that it's a new array, otherwise the overload could be ambiguous), and b) clashes with the existing new keyword

[jiribenes]

There's a draft in the fix/issue484 branch, but I think we want to wait until #469 is merged.
See #488.