Maps without keys should be given a 64-bit integer key of 0
Maps without keys should be given a 64-bit integer key of 0

Previously the auto-generated key type was the same as the value type,
which made maps containing string values use far more stack memory than
they should when accessing them.

ajor/bpftrace,ajor/bpftrace,iovisor/bpftrace,ajor/bpftrace,iovisor/bpftrace,iovisor/bpftrace,iovisor/bpftrace