Add struct_eq builtin for member-wise struct comparisons

[NotsoanoNimus]

Allows comparisons of input structures that don't already have an == overload (and it will just compare them anyway if they do). This reduces the need to define @operator(==) overrides which just do return self.a == self.a && self.b == self.b && ...;. They can now become something like this:

struct MyType
{
    bool has_field;
    uint128 something;
    String[] list;
}
// This...
fn bool MyType.old_eq(&self, MyType other) @operator(==)
{
    return self.has_field == other.has_field && self.something == other.something && self.list[..] == other.list[..];
}
// ... now becomes:
fn bool MyType.eq(&self, MyType other) @operator(==) => struct_eq(*self, other);

This is especially useful when memcmp-style comparisons of structs are not.

You can also use the variadic arguments list to supply strings in dot-notation to exclude certain fields from being compared. See the unit tests for more information about how to do this.

The implementation is a bit redundant because there are no flow-control statements at compile-time. This could certainly use a few extra reviewers to ensure it works as broadly as possible, but tests seem to show a good coverage/capability.