add initial dis utility

So to build it needs `-pkg cmdliner`, but you said not to add it to bapbuild (and so I didn't add it to oasis). Please comment on any style things that come up, e.g. I don't know if `@@` or `(foo bar)` styles are better. You will probably spot many things :)

Currently
```
OPTIONS
       -a ARCH, --arch=ARCH (absent=x86_64)
           Target architecture.

       -c, --code
           Output instruction code.

       --help[=FMT] (default=pager)
           Show this help in format FMT (pager, plain or groff).

       -k, --kinds
           Output instruction kinds.

       -p, --ops
           Output instruction opcode information.
```

Example inputs:
```
echo "0x48 0x83 0xec 0x08 0xe8 0x47 0xee 0xff 0xff 0x8b 0x40 0x10 0x48 0x83 0xc4 0x08 0xc3" | ./dis.native -pck
echo "\x48\x83\xec\x08\xe8\x47\xee\xff\xff\x8b\x40\x10\x48\x83\xc4\x08\xc3" | ./dis.native -pck
echo "48 83 ec 08 e8 47 ee ff ff 8b 40 10 48 83 c4 08 c3" | ./dis.native -pck

echo "c5 01 30 43" | ./dis.native --arch=arm -pck
echo "\xc5\x01\x30\x43" | ./dis.native -a arm -pck
echo "c5 01 30 43" | ./dis.native -aarm -pck
```

Example output (by the way tab width is greater on terminal, will look a little different (extra information columns are spaced farther away from disassembly)
x86_64
```
		subq	$8, %rsp
						|insn:2691
						|reg:RSP:44
						|reg:RSP:44
						|imm:8
						|kinds:may_load, may_store
		callq	-4537
						|insn:390
						|imm:-4537
						|kinds:call, may_affect_control_flow, may_load, may_store
		movl	16(%rax), %eax
						|insn:1579
						|reg:EAX:19
						|reg:RAX:35
						|imm:1
						|reg:nil:0
						|imm:16
						|reg:nil:0
						|kinds:may_load, may_store
		addq	$8, %rsp
						|insn:99
						|reg:RSP:44
						|reg:RSP:44
						|imm:8
						|kinds:may_load, may_store
		ret
						|insn:2321
						|kinds:return, barrier, terminator, may_affect_control_flow, may_load, may_store
```
Other thought: kinds can wrap-around on terminal, maybe there's a better way to represent it...

ARM
```
		teqmi	r0, #1073741873
						|insn:513
						|reg:R0:66
						|imm:1073741873
						|imm:4
						|reg:CPSR:3
						|kinds:may_load, may_store
```

I did have something that checked List.length before and nuked it, will change.

Why is char_of_int cheating? Because it's not core?