Wrong layout in blame error message involving records

[yannham]

Describe the bug
When showing a blame error for a record contract, the underlining that points at the problematic field is off. Likely, #182 introduced a new simplified row syntax, but the code that determines the positioning of the underline and the accompanying label has not been updated accordingly.

To Reproduce

$nickel <<< 'deepSeq Assume({ a: { b: Num } }, {a = {b = "a"}}) 1'
error: Blame error: contract broken by a function.
  ┌─ :1:14
  │
1 │ {a: {b: Num}}
  │                expected field type
  │

Expected behavior
Have expected field type message rightly positioned with a line under Num:

error: Blame error: contract broken by a function.
  ┌─ :1:9
  │
1 │ {a: {b: Num}}
  │         --- expected field type
  │