debugger: add a lua util for dumping memory.

Get nice hexdumps like:

oldland> dump_mem(0x00000000, 64)
00 00 00 ee 00 00 40 f4 01 00 00 82 00 01 00 30    |......@........0|
fd ff ff 58 00 00 00 ec 00 30 01 f4 01 00 00 ec    |...X.....0......|
11 68 01 f4 03 00 80 ec 10 00 00 32 05 00 00 58    |.h.........2...X|
02 00 00 8a 20 03 00 9a 00 10 00 00 03 13 00 00    |.... ...........|

and

oldland> dump_mem(0x20000000, 32)
48 65 6c 6c 6f 2c 20 77 6f 72 6c 64 21 00 f0 0f    |Hello, world!...|
f0 0f f0 0f f0 0f f0 0f f0 0f f0 0f f0 0f f0 0f    |................|