Allow parser to take optional secondary std.mem.Allocator parameter

[maxbol]

Not sure if this would be interesting for merging into the main tree, but I wrote a little wrapper to allow parsing functions to take the parser allocator as a secondary parameter. Potential use case: washing strings, reordering lists etc.

Of course the pitfall here is that the main program must remember to deallocate these values at some point.

Parsers are still allowed to take ONLY the value parameter. Not sure how zig-ish this is, but at least it doesn't break backward compat.

Merge only if this speaks to you, and lmk if there is something I clean up/rename/make more idiomatic (not a prof zig developer by any means :))

fn parseLowercaseString(value: []const u8, allocator: std.mem.Allocator) ![]const u8 {
    return std.ascii.allocLowerString(allocator, value);
}

fn parseSomethingElse(value: u32) i32 {
    // Still allowed to only take the value as parameter
    ...
}

...

const params = comptime clap.parseParamsComptime(
    \\--to-lower <raw_str>
);

const parsers = comptime {
    .raw_str = parseLowercaseString,
};

[maxbol]

Removed nix crud that was added by mistake

[Hejsil]

Yea, giving an allocator to the value parsers is pretty useful. Instead of allowing both signature fn ([]const u8) !T and fn (std.mem.Allocator, []const u8) !T, I'd prefer to always just take the latter. If the function doesn't need the allocator argument it can remove it.

[maxbol]

Sounds just fine by me but also like it's going to break current consumers with custom parsers. I'll stick to my own branch for now and let you cook on this 🔥