Investigate using an arena allocator

[wincent]

Probably not useful but check out isolated malloc calls we're doing to see if it would help. (I think our pervasive use of slab allocations already extracts all the benefit we could expect to receive.)

Random search results describing arena allocators:

• https://en.wikipedia.org/wiki/Region-based_memory_management
• https://ziap.github.io/blog/arena/
• https://www.rfleury.com/p/untangling-lifetimes-the-arena-allocator
• https://github.com/shantanu/arena-allocator

See also:

• perf: add experimental support for using mimalloc allocator #404