Create more accurate global allocation pool defintion

[ALSchwalm]

Currently the global allocator is allocated from a region that is found by the following algorithm:

1. Get the range of available addresses from the multiboot info
2. Get the range of excluded addresses (because they are occupied by other modules, elf sections, etc)
3. Allocate from the portion of the available addresses that is past the end of the maximum excluded address

This is obviously not correct. If, for example, there is an excluded address at the end of the available regions, this will fail. It is also not guaranteed to produce the largest available range.

See https://github.com/mythril-hypervisor/mythril/blob/master/mythril_multiboot2/src/main.rs#L128

[ALSchwalm]

This probably relates to #85

[ALSchwalm]

When this is done, it would be good to develop a series of tests as well. It seems like that should be pretty doable.