Fix Array#unshift for large arrays

[HertzDevil]

#unshift still assumes the new capacity is exactly twice the old one whenever a reallocation is needed, which is no longer true for large arrays after #11482, so everything past around the first new_capacity / 2 elements would be referring to invalid storage, leading to potential heap corruption:

x = Array.new(300, &.itself)
x.@capacity # => 300
x           # => [0, 1, 2, ..., 282, 283, 284, 285, ..., 298, 299]

x.unshift 1000
x.@capacity # => 567
x           # => [1000, 0, 1, 2, ..., 282, 283, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0]

x = Array.new(300) { [] of Int32 }
x.unshift [1]
x # => [[1], [], [], ..., [Invalid memory access (signal 11) at address 0x4

Pushes will also no longer reallocate:

x = Array.new(300, &.itself)
x.@capacity # => 300
x.size      # => 300
x.unshift 1000
x.@capacity # => 567
x.size      # => 301
50000.times { x << 2000 }
x.@capacity # => 567
x.size      # => 50301

This PR fixes it.

[straight-shoota]

Why not test the actual value?

[beta-ziliani]

Thanks!