Add heapless-backed ByteBuf

[ilmai]

Use of alloc crate is now behind a feature

Still missing proper error handling, but I wanted to get an idea if this would be a potentially accepted PR before putting in too much effort.

[bochaco]

I'm also interested in such a feat as I'm tying to use this crate from an embedded app (a Ledger embedded app).

@ilmai perhaps it'll be better to split buffer.rs into two files for diff impls?

[bochaco]

Also, it seems having PR #334 merged will be beneficial for this.