Use a bump / arena memory allocator

[ahdinosaur]

Is your feature request related to a problem? Please describe.
Since Rimu is a template language, all the objects allocated during evaluation can be de-allocated after the result is returned (serialized). This means no need for a garbage collector and no need even for a standard heap allocator. Rimu may benefit from a bump / arena allocator.

Describe the solution you'd like
Investigate using bumpaloo, or another bump / arena allocator.

Describe alternatives you've considered

Additional context