Panic in 32bit environment

[PegasisForever]

I'm trying to use galloc on a 32bit arm microcontroller. When initializing, the library panicked at 'attempt to subtract with overflow' at this line.

I did some debugging and I think I found the reason:

• MIN_ALIGNMENT_LOG2 is the trailing zeros of MIN_ALIGNMENT, which has a minimum of 8.
• largest_power_of_2_that_divides_addr comes from addr + HEADER_SIZE, which addr is aligned but HEADER_SIZE is not aligned due to its use of usize in the 32bit environment.
• This causes 2 - 3 and triggers the panic.

I'd love to help testing the crate again if you make changes.