mem: get clever with memeqzero().

mem: get clever with memeqzero().

Best of both worlds.

Before:
1: 6ns
2: 7ns
4: 7ns
8: 7ns
16: 7ns
32: 8ns
64: 9ns
128: 13ns
256: 24ns
512: 47ns
1024: 92ns
2048: 185ns
4096: 376ns
8192: 739ns
16384: 1463ns
32768: 2914ns
65536: 5800ns
2: 7ns
3: 7ns
5: 7ns
9: 7ns
17: 7ns
33: 8ns
65: 9ns
129: 20ns
257: 31ns
513: 49ns
1025: 96ns
2049: 189ns
4097: 381ns
8193: 745ns
16385: 1477ns
32769: 2930ns
65537: 5824ns
total = 599391004

After:
1: 3ns
2: 3ns
4: 4ns
8: 5ns
16: 12ns
32: 13ns
64: 15ns
128: 19ns
256: 25ns
512: 35ns
1024: 57ns
2048: 105ns
4096: 183ns
8192: 324ns
16384: 607ns
32768: 1317ns
65536: 2774ns
2: 3ns
3: 3ns
5: 4ns
9: 6ns
17: 11ns
33: 13ns
65: 14ns
129: 19ns
257: 24ns
513: 35ns
1025: 57ns
2049: 106ns
4097: 183ns
8193: 324ns
16385: 607ns
32769: 1315ns
65537: 2773ns
total = 599391004

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>