Un-revert heapless change

[jamesmunns]

In #115, we updated heapless. I merged this assuming I was going to either do tricky feature stuff, or release a breaking change. That never ended up happening.

After that, we merged a couple of other quality of life items, so we reverted this change in #164.

Eventually we need to support a newer version of heapless.

[LeoniePhiline]

Would you be fine with re-exporting heapless?

This circumvents trouble with keeping versions of heapless in sync:

Without re-export, library users must pay extra attention to updating postcard and heapless in lockstep.