A replacement global allocator for wasm. It uses a linked list of 64KB
pages for large allocations, and a linked list of SimpleAllocators for
smaller ones. (Each SimpleAllocator can manage up to 2MB.)
Simple memory allocator. It manages a region of memory defined by either
an array or a pair of pointers (begin, end), and hands out blocks in
(wasm-sized, 32 bit) words, using a bump pointer and a free list. This is
a bog-standard algorithm I remember from school in the 1990s, so nothing
fancy.