Support allocating 0 bit types

[pfgithub]

This is useful for generic functions so 0 bit types don't have to be special cased every time. The current code errors because *const T cannot be assigned to *T.

[andrewrk]

I think this used to work, and regressed with some recent changes. Would you mind adding a small test to make sure it doesn't regress again?

[pfgithub]

stage1 doesn't really seem to like having a slice of 0 bit pointers very much

    var zero_bit_slice = try allocator.alloc(*u0, 0);
    testing.expect(zero_bit_slice.len == 0);

    zero_bit_slice = try allocator.realloc(zero_bit_slice, 100);
    testing.expect(zero_bit_slice.len == 100);

    for (zero_bit_slice) |*item| {
        item.* = try allocator.create(u0);
        item.*.* = 0;
    }
    for (zero_bit_slice) |item| {
        allocator.destroy(item);
    }
    allocator.free(zero_bit_slice);

Assertion failed at /deps/zig/src/analyze.cpp:615 in get_pointer_to_type_extra2. This is a bug in the Zig compiler.
Unable to dump stack trace: debug info stripped
fish: “zig test test.zig” terminated by signal SIGABRT (Abort)

            assert(byte_alignment == 0);
            entry->abi_size = 0;
            entry->size_in_bits = 0;
            entry->abi_align = 0;

I guess the byte alignment of *u0 is being set to 1, and it can't be manually set

./test.zig:36:53: error: alignment must be >= 1
    var zero_bit_slice = try allocator.alloc(*align(0) u0, 0);
                                                    ^