Refactor Indexable::Mutable#fill's overloads

[HertzDevil]

This PR reworks Indexable::Mutable#fill's overloads so that they follow the convention of other subrange-accepting methods in the Crystal standard library.

• The following overloads are now available in Indexable::Mutable(T):
  • #fill(value : T, start : Int, count : Int)
  • #fill(value : T, range : Range)
  • #fill(start : Int, count : Int, & : Int32 -> T)
  • #fill(range : Range, & : Int32 -> T)
• Array(T)#fill(value : T, start : Int) and Array(T)#fill(start : Int, & : Int32 -> T) are now deprecated. They are redundant because the Range overloads take care of them with a start.. argument; no other method in the standard library has such an overload on top of start + count and range ones.
• Having start == size or start + count > size no longer raise an exception. #fill will now replace fewer than count elements in that case (possibly zero). This is also the behaviour in Ruby.
• The non-block overloads in Array and StaticArray delegate to their underlying Slices to take advantage of the memset optimization. This includes also the subrange-accepting overloads.