Instead of parsing dbg with an expression block, parse the dbg keyword
with no additional arguments. This way the parser treats dbg just like a
variable in function application. We desugar by pattern matching on
`Apply(Dbg, args, called_via)` nodes. This changes the output of syntax
tests since the initial AST is different, but does not change the output
of can or mono.
Add two new errors for dbg in expression position with either no args or
too many args. This is similar to the error behavior of `crash`.
Continue to parse dbg statements with an expression block, as before.
Make `Expr::Dbg` node specific to dbg expressions, add `Expr::DbgStmt`
for dbg statements. Debug expressions don't have a continuation branch.
Support formatting both kinds of `Dbg` nodes.
