optimize size of object header

[soc]

The header takes up 128 bits (2 pointers á 64 bits) currently, which is too much.

There are various ways to reduce this, ideally to 64 bits or fewer:

Reducing number of pointers:

• using a garbage collector that does not require two pointers

Reducing size of individual pointers:

See https://soc.me/runtimes/header-compression.