Add support for field-local %error handlers.

[rsmmr]

Add support for field-local %error handlers.

We now support attaching an %error handler to an individual field:

    type Test = unit {
        a: b"A";
        b: b"B" %error { print "field B %error", self; }
        c: b"C";
    };

With input AxC, that handler will trigger, whereas with ABx it
won't. If the unit had a unit-wide %error handler as well, that one
would trigger in both cases (i.e., for b, in addition to its field
local handler).

The handler can also be provided separately from the field:

    on b %error { ... }

In that separate version, one can receive the error message as well by
declaring a corresponding string parameter:

    on b(msg: string) %error { ... }

This works externally, from outside the unit, as well:

    on Test::b(msg: string) %error { ... }

This is debased on top of topic/robin/optimize-type-parsing so that
we get the peephole optimizer.

Addresses #1824.

[rsmmr]

@sethhall another one to try

[sethhall]

This doesn't seem to work if you have any attributes attached to the field...

type Test1 = unit {
    a: b"A";
    b: bytes &size=1 %error { print "field B %error", self; }
    c: b"C";
};

[error] test17.spicy:5:23-5:27: unknown ID 'error'
[error] spicyc: aborting after errors

[rsmmr]

b: bytes &size=1 %error { print "field B %error", self; }

Turns out this gets parsed as &size=(1 % error) (i.e., a module expression) ... !

[rsmmr]

Turns out this gets parsed as &size=(1 % error) (i.e., a module expression) ... !

... and this is because in the past we have explicitly added support for parsing things like l %k == 10; as a valid expression.

[sethhall]

Turns out this gets parsed as &size=(1 % error) (i.e., a module expression) ... !

haha, well I'm glad I bumped across that when I tried to use the feature!

[bbannier]

Thanks! 🚀