I am hitting many error recovery problems. This is an attempt to make
auto-completion more robust.
For example, we currently can't auto complete inside of parentheses.
The syntax tree for the syntactically incorrect tern `(s. , 0)` is
```
(Term.paren "(" [(Term.proj `s ".")])
```
The elaborator for `paren` fails to match this partial syntax tree and
returns an error. I considered adding code to the `else` branch of
this elaborator and handle partial trees there. However, we would
still be losing information for the other elements of the tuple.
Example: no hover information for them.
The helper parser makes sure we don't lose information.
0717c3d0b2