Related to #100.

Would you consider addition of an unsafe method (called e.g. from_raw_parts) to create a Bump from an existing memory allocation?

In comparison to the request in #100, the caller would have to promise that the allocation was made from the global allocator, with the alignment that Bumpalo requires, and that this allocation now becomes owned by the Bump. So then no changes required to Bumpalo's internals.

One circumstance where this would be useful is Rust-Javascript interop, where it'd enable re-using the same Bump for each call into Rust, rather than calling into the global allocator to create a fresh one each time.