new syntax for user operator declarations and invocations

[mccanne]

This commit improves the syntax for user operators to better align with built-in operators and distinguish their syntax from user functions.

The new syntax for op declarations is:

 op name arg, arg, ... : ( ... )

and invoking an operator is now:

call name arg, arg, ...

with a short-cut where you can drop the "call" as a shortcut:

name arg, arg, ...

(as long as the shortcut is not a single operator with no arguments inside of a subquery)

Updated documentation is coming in a subsequent PR from the book branch.

[nwt]

Nit: We usually name these for the field they initialize rather than their rule.

Suggested change

	/ !CallIDGuard id:Identifier _ !CallExprGuard args:Exprs {
	return &ast.CallOp{Kind:"CallOp", Name: id.(*ast.ID), Args:sliceOf[ast.Expr](args), Loc:loc(c)}, nil
	}
	/ !CallIDGuard id:Identifier &EndOfOp {
	return &ast.CallOp{Kind:"CallOp", Name: id.(*ast.ID), Loc:loc(c)}, nil
	/ !CallIDGuard name:Identifier _ !CallExprGuard args:Exprs {
	return &ast.CallOp{Kind: "CallOp", Name: name.(*ast.ID), Args: sliceOf[ast.Expr](args), Loc: loc(c)}, nil
	}
	/ !CallIDGuard name:Identifier &EndOfOp {
	return &ast.CallOp{Kind: "CallOp", Name: name.(*ast.ID), Loc: loc(c)}, nil

[nwt]

Nits: Name the Identifier label for the field it initializes. And passing nil to sliceOf is safe.

Suggested change

	= CALL _ id:Identifier args:((_ args:Exprs) { return args, nil })? {
	op := &ast.CallOp{Kind:"CallOp", Name: id.(*ast.ID), Loc:loc(c)}
	if args != nil {
	op.Args = sliceOf[ast.Expr](args)
	}
	return op, nil
	= CALL _ name:Identifier args:((_ args:Exprs) { return args, nil })? {
	return &ast.CallOp{
	Kind: "CallOp",
	Name: name.(*ast.ID),
	Args: sliceOf[ast.Expr](args),
	Loc: loc(c)
	}, nil
	}

[nwt]

Nit: For consistency with the other items in this list.

Suggested change

	// a shortcut "call" operator, or an expression that is either an implied "values"
	// a shortcut "call", or an expression that is either an implied "values"

[nwt]

I think this new syntax looks really good!