std.meta.trait: mark all the functions as 'inline'

[wrongnull]

Currently there is nothing indicates that those functions should be called with explicit comptime. This can lead to strange error messages in user code such as following:

const std = @import("std");

const MyUnion = union(enum) {
    field1: u32,
    field2: [] const u8,
};


fn constructMyUnion(comptime arg: anytype) MyUnion {
    if (std.meta.trait.isIntegral(@TypeOf(arg))) {
        return .{ .field1 = arg };
    }
    return .{ .field2 = arg };
}

pub fn main() void {
    _ = constructMyUnion(1);
}

error: expected type '[]const u8', found 'comptime_int'

[mlugg]

Functions returning TraitFn need not be marked inline; they're already comptime-only.

[wrongnull]

[wrongnull]

I close this pr in favor of #18061