Remove vertex_index(::Integer), make special case for SimpleGraph

Thanks @sbromberger
Fixes #131
Fixes #127
Closes #133
I don't know what constitute's bad -- it (and `method_exists`) just doesn't specialize like I expected the first time I used it:
Using that sample code above:

```
code_native(a_or_b2, (A, Int))
	.section	__TEXT,__text,regular,pure_instructions
Filename: none
Source line: 2
	push	RBP
	mov	RBP, RSP
	push	R15
	push	R14
	push	R12
	push	RBX
	sub	RSP, 32
	mov	RBX, RSI
	mov	R14, RDI
	mov	QWORD PTR [RBP - 64], 4
Source line: 2
	movabs	R12, jl_pgcstack
	mov	RAX, QWORD PTR [R12]
	mov	QWORD PTR [RBP - 56], RAX
	lea	RAX, QWORD PTR [RBP - 64]
	mov	QWORD PTR [R12], RAX
	vxorps	XMM0, XMM0, XMM0
	vmovups	XMMWORD PTR [RBP - 48], XMM0
	movabs	RAX, 140405752947744
Source line: 2
	mov	RDI, QWORD PTR [RAX]
	mov	RAX, QWORD PTR [RDI + 8]
	movabs	RCX, 140405770133600
	mov	RCX, QWORD PTR [RCX]
Source line: 2
	lea	R15, QWORD PTR [RBP - 48]
	movabs	RDX, 140405903671888
Source line: 2
	mov	QWORD PTR [RBP - 48], RCX
	mov	QWORD PTR [RBP - 40], RDX
	mov	RSI, R15
	mov	EDX, 2
	call	RAX
	movabs	RCX, 140405753013296
	cmp	RAX, RCX
	je	L218
Source line: 3
	movabs	RAX, allocobj
	mov	EDI, 16
	call	RAX
	movabs	RCX, 140405864207744
	mov	QWORD PTR [RAX], RCX
	mov	QWORD PTR [RAX + 8], R14
	mov	QWORD PTR [RBP - 48], RAX
	movabs	RAX, jl_apply_generic
	movabs	RDI, 140405793447904
	mov	RSI, R15
	mov	EDX, 1
	call	RAX
	jmpq	L240
Source line: 5
L218:	imul	RBX, RBX
	add	RBX, RBX
	movabs	RAX, jl_box_int64
	mov	RDI, RBX
	call	RAX
L240:	mov	RCX, QWORD PTR [RBP - 56]
	mov	QWORD PTR [R12], RCX
	add	RSP, 32
	pop	RBX
	pop	R12
	pop	R14
	pop	R15
	pop	RBP
	ret
```

```
julia> code_native(a_only, (A, Int))
	.section	__TEXT,__text,regular,pure_instructions
Filename: none
Source line: 1
	push	RBP
	mov	RBP, RSP
Source line: 1
	imul	RSI, RSI
	lea	RAX, QWORD PTR [RSI + RSI]
	pop	RBP
	ret
```