Change Encoding parameter sizes

[madsmtm]

• Decrease Encoding::BitField size from u32 to u8
  • A bitfield can only be created from integral types, whose maximum size is 64 (which can easily fit inside a u8)
• Increase Encoding::Array length from u32 to usize
  • @encode can output an array size up to 2^61 - 1 elements:

  NSLog(@"Encoding: %s", @encode(char [((unsigned long long)(2) << 60) - 1]));

These are breaking changes.

Replaces SSheldon/rust-objc-encode#4.