Allow @private attribute for constant alias declarations

[marcs-feh]

When writing code, one common thing a programmer might wish to do is reference another function by a shorter name without making it a part of the public interface for their library/program. Odin disallows all attributes for constant alias declarations, I understand this decision makes sense for things like @static or @link_prefix. However, @private only refers to the name access aspect, and thus can be safely applied to an alias without "corrupting" the attributes of the aliased thing.

There's also 2 more design considerations to take:

The current system is inconsistent. here's an example:

import "base:intrinsics"
import "core:testing"

@private
cas :: intrinsics.atomic_compare_exchange_strong // This is allowed (good)

@private
expect :: testing.expect // Not allowed.

The current system can also be trivially circumvented for procedures either way but it's very ugly:

import "core:slice"

// @private find :: slice.linear_search
@private
find :: #force_inline proc(arr: $A/[]$T, key: T) -> (int, bool){
    return slice.linear_search(arr, key)
}