Document dynamic memory allocator trade-offs

[HCastano]

After use-ink/ink#831 is merged there will be two choices for dynamic memory allocators in ink!. The default is the new bump allocator while the other choice is wee_alloc (which used to be the old default).

The fundamental tradeoff here is .wasm size vs. efficient use of memory.

For someone concerned with running smart contracts on a parachain, size would be the most important consideration. However, if someone is running a standalone chain then memory efficiency might be more important.

The guide should explain:

• The role of a dynamic memory allocator
• The different allocators available
• The trade-offs between the allocators
• How to switch between allocators

[SkymanOne]

Do we still allow choosing an allocator?

[HCastano]

Yes we do.

I have a recent PR around this: use-ink/ink#1661.

If we want to add a section around this in the ink-docs it can basically be a copy-paste of the documentation I wrote in that PR.

[SkymanOne]

Yes. Can you please add it to the docs?

[HCastano]

No promises on the timelines though