Ran `cargo fmt` and renamed AllocErr to AllocError. The main rename
was updating the Alloc trait (now the Allocator trait) and
associated methods.

Includes more renaming missed in the previous commit.
The allocate method needs to return a NonNull<[u8]>, so this commit
updates bunch of callsites to handle the returned allocation correctly.
Also cleaned up some alignment asserts for consistency (0 is not a power
of 2, so some of the existing asserts were extraneous).