Improve the default allocator

[connortsui20]

I don't like that we even expose is_statically_allocated (this was me...). But we shouldn't be switching on the implementation of the allocator imo.

We already do this somewhere, so let's keep it and remove this when we experiment with new buffer allocators.

Originally posted by @gatesn in #10030 (comment)

[connortsui20]

Also related: #9812

Might just want to hold the Layout next to the Buffer