mem: add splitBackwards

[motiejus]

Over the last couple of weeks weeks I needed to iterate over a
collection backwards at least twice. Do we want to have this in stdlib?
If yes, click "Merge" and start using today! Free shipping and returns
(before 1.0).

Why is this useful?

I need this for building an error wrapper: errors are added in the
wrapper from "lowest" level to "highest" level, and then printed in
reverse order. Imagine UpdateUsers call, which needs to return
error.InvalidInput and a wrappable error context. In Go we would add a
context to the error when returning it:

// if update_user fails, add context on which user we are operating
if err := update_user(user); err != nil {
    return fmt.Errorf("user id=%d: %w", user.id, err)
}

Since Zig cannot pass anything else than u16 with an error (#2647), I
will pass a err_ctx: *Err, to the callers, where they can, besides
returning an error, augment it with auxiliary data. Err is a
preallocated array that can add zero-byte-separated strings. For a
concrete example, imagine such a call graph:

update_user(User, *Err) error{InvalidInput}!<...>
  validate_user([]const u8, *Err) error{InvalidInput}!<...>

Where validate_user would like, besides only the error, signal the
invalid field. And update_user, besides the error, would signal the
offending user id.

We also don't want the low-level functions to know in which context they
are operating to construct a meaningful error message: if validation
fails, they append their "context" to the buffer. To translate/augment
the Go example above:

pub fn validate_user(err_ctx: *Err, user: User) error{InvalidInput}!void {
    const name = user.name;
    if (!ascii.isAlpha(user.name)) {
        err_ctx.print("name '{s}' must be ascii-letters only", .{user.name});
        return error.InvalidInput;
    }
    <...>
}

// update_user validates each user and does something with it.
pub fn update_user(err_ctx: *Err, user: User) error{InvalidInput}!void {
    // validate the user before updating it
    validate_user(user) catch {
        err_ctx.print("user id={d}", .{user.id});
        return error.InvalidInput;
    };
    <...>
}

Then the top-level function (in my case, CLI) will read the buffer
backwards (splitting on "\x00") and print:

user id=123: name 'Žvangalas' must be ascii-letters only

To read that buffer backwards, dear readers of this commit message, I
need mem.splitBackwards.

This PR has 2 commits:

1. refactor split tests to use assertEqualSlices.
2. refactor split tests to use rest() occasionally.
3. add splitBackwards with similar tests to split.

[InKryption]

Function names are expected to be camelCase: https://ziglang.org/documentation/master/#Style-Guide

[motiejus]

That's what happens when you need to change the programming language more than once a day.

[motiejus]

Looks like github is having trouble with its cache: I renamed split_rev to splitRev in both title and description, and it is still not reflected in the UI.

[Luukdegram]

I don't want to get into bikeshedding territory but I wonder if it should be named splitReversed fully written out, similarly to how we have copyBackwards.

[motiejus]

I don't want to get into bikeshedding territory but I wonder if it should be named splitReversed fully written out, similarly to how we have copyBackwards.

I like the color of your bike shed better; updating to splitReversed.

[motiejus]

I don't want to get into bikeshedding territory but I wonder if it should be named splitReversed fully written out, similarly to how we have copyBackwards.

Repainted again, so splitBackwards is now consistent with copyBackwards.