Tracking issue: consider allocating Blobs on the heap

[kevaundray]

Currently Blobs are allocated on the stack in some of the bindings, see for example, Rust here

This is a tracking issue to discuss changing it so that Blobs are heap allocated, where the benefit is that we do not put large (131KB per Blob) items on the stack, consequently removing the possibility of accidental stackoverflow issues in downstream dependencies.

[kevaundray]

(Same argument could apply for the Cell type too)