Make the concrete syntax more flexible about missing parenthesis
Make the concrete syntax more flexible about missing parenthesis

The concrete syntax will now permit:
`a + b : num : num + c + let x = 3 in x + y`
inserting parenthesis during parsing to make it
`a + ((b : num) : num) + c + (let x = 3 in x + y)`

hazelgrove/hazel,hazelgrove/hazel,hazelgrove/hazel