Change Pair to use fst/snd annotations

[krame505]

Changes

Depends on #773.

We discussed this change a while ago but never actually did it - this changes the definition of Pair from

synthesized attribute fst<a> :: a;
synthesized attribute snd<a> :: a;
data nonterminal Pair<a b> with fst<a>, snd<b>;
production pair
top::Pair<a b> ::= x::a y::b
{
  top.fst = x;
  top.snd = y;
}

to

annotation fst<a> :: a;
annotation snd<a> :: a;
data nonterminal Pair<a b> with fst<a>, snd<b>;
production pair
top::Pair<a b> ::=
{}

This gives a slight performance improvement, as we don't need to create a DecoratedNode to access an annotation. This is also a nice demo of Silver's support for "records" via annotations (which works out a bit nicer now with the addition of deriving support in #774.)

Documentation

https://github.com/melt-umn/melt-website/pull/52

[krame505]

This is currently failing to build due https://github.com/melt-umn/silver/issues/776

[dpulls[bot]]

:tada: All dependencies have been resolved !

[ericvanwyk]

Can one still pattern match on pairs if annotations are used instead of regular child terms?

[krame505]

Yes, one can still write

case foo of
| pair(fst=42, y=a) -> ...
end

or

case foo of
| (42, a) -> ...
end

using tuple syntax.

[ericvanwyk]

Of course - the tuples extension is what we'd use in most applications. Thanks.