slicing syntax which can obtain sentinel terminated types from non-sentinel-terminated types

[andrewrk]

Moved from #3731.

const std = @import("std");
const assert = std.debug.assert;

test "obtaining a null terminated slice" {
    // here we have a normal array
    var buf: [50]u8 = undefined;

    buf[0] = 'a';
    buf[1] = 'b';
    buf[2] = 'c';
    buf[3] = 0;

    // now we obtain a null terminated slice:
    const ptr = buf[0..3 :0];

    // ptr is a pointer to null-terminated array,
    // because the len was comptime known (See #863)
    comptime assert(@typeOf(ptr) == *[3:0]u8);

    var runtime_len: usize = 3;
    const ptr2 = buf[0..runtime_len :0];
    // ptr2 is a null-terminated slice
    comptime assert(@typeOf(ptr2) == [:0]u8);


    buf[3] += 1;
    _ = buf[0..3 :0]; // safety panic: slice sentinel assertion failed
}

Along with this:

• slicing a sentinel-terminated pointer without this syntax should yield a non-sentinel-terminated result.
• index of a sentinel-terminated array which is comptime-known to be len, is a comptime-known value when loading. no-op with safety check when storing. extracted to improved handling of index of a sentinel-terminated array which is comptime-known to be len #3962
• compile error for incorrect sentinel when doing sentinel slicing extracted to add compile error for sentinel assertion failed #3963

There are already quite a few places this would be handy:

../lib/std/fs.zig:    // TODO avoid @ptrCast here using slice syntax with https://github.com/ziglang/zig/issues/3731
../lib/std/os.zig:        // TODO avoid @ptrCast here using slice syntax with https://github.com/ziglang/zig/issues/3731
../lib/std/os.zig:        // TODO avoid @ptrCast using slice syntax with https://github.com/ziglang/zig/issues/3731
../lib/std/os.zig:    // TODO avoid @ptrCast here using slice syntax with https://github.com/ziglang/zig/issues/3731
../lib/std/os.zig:            // TODO avoid @ptrCast here using slice syntax with https://github.com/ziglang/zig/issues/3731
../lib/std/os.zig:    // TODO avoid @ptrCast here using slice syntax with https://github.com/ziglang/zig/issues/3731

[JesseRMeyer]

Nit:

// ptr2 is a null-terminated slice
comptime assert(@typeOf(ptr) == [:0]u8);

Should be:

// ptr2 is a null-terminated slice
comptime assert(@typeOf(ptr2) == [:0]u8);

[daurnimator]

Would be interesting to use this for e.g. newline terminated strings.