libcore: assume the input of `next_code_point` and `next_code_point_reverse` is UTF...

libcore: assume the input of `next_code_point` and `next_code_point_reverse` is UTF-8-like

The functions are now `unsafe` and they use `Option::unwrap_unchecked` instead of `unwrap_or_0`

`unwrap_or_0` was added in 42357d772b8a3a1ce4395deeac0a5cf1f66e951d. I guess `unwrap_unchecked` was not available back then.

Given this example:

```rust
pub fn first_char(s: &str) -> Option<char> {
    s.chars().next()
}
```

Previously, the following assembly was produced:

```asm
_ZN7example10first_char17ha056ddea6bafad1cE:
	.cfi_startproc
	test	rsi, rsi
	je	.LBB0_1
	movzx	edx, byte ptr [rdi]
	test	dl, dl
	js	.LBB0_3
	mov	eax, edx
	ret
.LBB0_1:
	mov	eax, 1114112
	ret
.LBB0_3:
	lea	r8, [rdi + rsi]
	xor	eax, eax
	mov	r9, r8
	cmp	rsi, 1
	je	.LBB0_5
	movzx	eax, byte ptr [rdi + 1]
	add	rdi, 2
	and	eax, 63
	mov	r9, rdi
.LBB0_5:
	mov	ecx, edx
	and	ecx, 31
	cmp	dl, -33
	jbe	.LBB0_6
	cmp	r9, r8
	je	.LBB0_9
	movzx	esi, byte ptr [r9]
	add	r9, 1
	and	esi, 63
	shl	eax, 6
	or	eax, esi
	cmp	dl, -16
	jb	.LBB0_12
.LBB0_13:
	cmp	r9, r8
	je	.LBB0_14
	movzx	edx, byte ptr [r9]
	and	edx, 63
	jmp	.LBB0_16
.LBB0_6:
	shl	ecx, 6
	or	eax, ecx
	ret
.LBB0_9:
	xor	esi, esi
	mov	r9, r8
	shl	eax, 6
	or	eax, esi
	cmp	dl, -16
	jae	.LBB0_13
.LBB0_12:
	shl	ecx, 12
	or	eax, ecx
	ret
.LBB0_14:
	xor	edx, edx
.LBB0_16:
	and	ecx, 7
	shl	ecx, 18
	shl	eax, 6
	or	eax, ecx
	or	eax, edx
	ret
```

After this change, the assembly is reduced to:

```asm
_ZN7example10first_char17h4318683472f884ccE:
	.cfi_startproc
	test	rsi, rsi
	je	.LBB0_1
	movzx	ecx, byte ptr [rdi]
	test	cl, cl
	js	.LBB0_3
	mov	eax, ecx
	ret
.LBB0_1:
	mov	eax, 1114112
	ret
.LBB0_3:
	mov	eax, ecx
	and	eax, 31
	movzx	esi, byte ptr [rdi + 1]
	and	esi, 63
	cmp	cl, -33
	jbe	.LBB0_4
	movzx	edx, byte ptr [rdi + 2]
	shl	esi, 6
	and	edx, 63
	or	edx, esi
	cmp	cl, -16
	jb	.LBB0_7
	movzx	ecx, byte ptr [rdi + 3]
	and	eax, 7
	shl	eax, 18
	shl	edx, 6
	and	ecx, 63
	or	ecx, edx
	or	eax, ecx
	ret
.LBB0_4:
	shl	eax, 6
	or	eax, esi
	ret
.LBB0_7:
	shl	eax, 12
	or	eax, edx
	ret
```