Improve BitArray's constructors

[HertzDevil]

• Adds a block-accepting overload, similar to other containers: BitArray.new(7, &.even?) # => BitArray[1010101]
• Allows non-Int32 size arguments, also similar to other containers. The size is converted within the constructor. The added Int restriction is not a breaking change because size was previously used to initialize an Int32 instance variable directly.
• Raises if the size argument is negative, instead of silently returning an empty BitArray for sizes within -31..-1.

I avoided BitArray.[] because it could be interpreted in two ways, a variadic literal-like constructor (which requires changes to #inspect's format), and an integer-to-bits constructor (similar to the from_int spec helper method).

[straight-shoota]

I'm not sure if the yielding overload is that useful, but it's probably good to have it for consistency.