Allow for array sizing using consts

[cujomalainey]

Consts can't be used for array sizing. e.g.

const SIZE: usize = 4;

#[derive(PackedStruct)]
#[packed_struct(bit_numbering = "msb0", endian = "lsb", size_bytes = "4")]
struct A {
  b: [u8; SIZE],
}

Which returns error: Unsupported extraction of int value

[jkorinth]

👍